Name: Sourdough Peanut Butter Squares
Prep time: 15 min
Cook time: 40 min
Total time: 55 min
Ingredients: 120 g Peanut Butter ((see notes)); 100 g Butter ((softened at room temperature)); 130 g White Sugar; 70 g Brown Sugar; 2 Eggs ((mine weighed 70g each, use 3 if yours are smaller)); 5 g Vanilla Paste; 120 g Sourdough Starter; 120 g All Purpose Flour; 3 g Baking Powder; 3 g Salt; 100 g Chocolate Chips ((optional)); 110 g Peanut Butter

If you're a peanut butter lover with a jar of sourdough starter on your counter, then you need to drop everything and bake these sourdough peanut butter squares! They're a little bit like a blondie in texture, but with oodles of peanut butter flavor! They're almost like a brownie, but I don't think they're fudgy enough to be called a brownie.

You know the best way to find out what they taste like? Bake them! So let's go!

9 sourdough peanut butter squares topped with crushed peanuts and set in front of a stack of white plates topped with cake forks.

Why You'll Love This Recipe!

Big on Flavor - If you love all things peanut butter, then you are going to love these. They have peanut butter in the batter plus more peanut butter swirled on top before baking.

Not Chocolate - many peanut butter desserts and cakes feature chocolate, which is great, but these sourdough peanut butter bars focus solely on the peanut butter flavor you love, without the chocolate! If you want the combination of chocolate and peanut butter, then you need to bake these sourdough peanut butter brownies.

Dessert or Snack - These sourdough peanut butter squares are so versatile! You can eat them as a snack, for dessert (like I've done with ice cream and chocolate sauce below) or they're even passable for breakfast (peanut butter is protein, right?)!

A sourdough peanut butter square on a white plate topped with vanilla ice cream, chocolate sauce and crushed peanuts. There is a fork taking a piece of the peanut butter square.

Ingredients

Peanut Butter - I've used a crunchy, natural peanut butter without added sugar or oils, but you can use smooth if you prefer. Just make sure you stir it well before weighing to ensure an even consistency.
Butter - softened butter blends more easily with sugar, creating a smooth, creamy base for the batter. Avoid melting it, as that can change the texture of the squares. As usual, I've used salted butter.
Sugar - I've used a combination of both brown and white sugar to create these squares. The white sugar creates the crispy edges, while the brown sugar gives the squares a deeper, more caramelised flavor.
Eggs - Eggs bind the ingredients and add moisture. I've used large eggs from my backyard chickens weighing around 70g each. If your eggs are smaller, I recommend using 3.
Vanilla Paste - adds a concentrated, rich vanilla flavor and those lovely specks. You can substitute with vanilla extract if that’s what you have on hand.
Sourdough Starter - use discard or active starter, either works here. It adds depth of flavor and a soft, tender texture without tasting overly sour. I've used active starter when creating this recipe, however discard is absolutely fine.
All Purpose Flour - better to use all purpose flour than bread flour here for a more tender crumb.
Baking Powder - just like making a brownie, you don't want too much baking powder so make sure you measure this accurately.
Salt - A pinch of salt balances the sweetness and enhances the peanut butter flavor. You might want to adjust this is if you are using salted peanut butter.
Chocolate Chips (optional) - stir in chocolate chips for extra indulgence. Semi-sweet or dark chips work best to complement the peanut butter richness.

How To Make Sourdough Peanut Butter Bars

I find it easiest to use a stand mixer to make the first part of this recipe, but you can also use an electric hand mixer or even a whisk if you want to.

Preheat the oven to 150C. Line a 9" x 9" brownie pan with parchment paper (you can spray with cooking oil or grease with butter if you prefer).

In the bowl, beat together peanut butter and butter until creamy (I use my stand mixer with the paddle attachment).

Next, add the white sugar, brown sugar, eggs, vanilla paste and sourdough starter. Beat until lighter in color and fluffy in texture. This takes around 2 minutes using the paddle attachment on my stand mixer.

Wet ingredients for making sourdough peanut butter squares that have been whipped until light and fluffy in a stand mixer.

Now, on top of the wet ingredients, add the all purpose flour, baking powder, salt and chocolate chips (if you're adding). Gently fold them into the wet ingredients using a spatula until the dry ingredients are just moistened. It's really important not to over mix here.

Wet and dry ingredients mixed together in a glass bowl.

Now, spoon the mixture into your prepared pan (it should be too thick to pour). Use your spatula to spread it out evenly across the base of the pan.

Melt the peanut butter in the microwave for 30 seconds. Stir it around so that it is fully melted and you can drizzle it. Now drizzle it over the peanut butter bar mixture in the pan. Use the end of a skewer to swirl it through the batter.

A series of 4 images showing how to add the sourdough peanut butter squares batter to the pan and swirl the peanut butter on the top.

Place the sourdough peanut butter bars into the oven for 30 to 40 minutes.

The sourdough peanut butter bars are done when the top is set, the edges are starting to crisp but the middle is still gooey when you shake the pan.

Take it out of the oven and allow it to cool in the pan for around an hour before slicing.

What Else Can You Put In Them?

I love these sourdough peanut butter squares just as they are, plain and simple. I serve them with hot coffee for an easy breakfast treat or morning pick me up. They are always met with enthusiasm from all members of my family! But there are also so many ways to jazz these up a little, and if I have a few bits and pieces on hand, I'll often throw some extras in. Here are some of the additions I make to create an extra special sourdough peanut butter square:

100g chocolate chips - you can add white chocolate, semi-sweet or dark chocolate depending on what you like. Double the choc chips to 200g if you want them extra sweet!
100g dates - I chop the dates up into at least quarters before adding them. This is my husband's favorite!
100g raisins or sultanas - I love adding these for some extra pops of sweetness.
100g peanut butter chips - these are the ultimate peanut butter indulgence and make these extra peanut buttery! I use Reese's Peanut Butter Chips if I'm doing this.

How To Store + Freeze?

A bit like sourdough brownies, I find that these sourdough peanut butter squares are actually better the next day. They almost need overnight to set once they're baked. Once baked and cooled, these can be stored in a airtight container for up to 5 days (if they even last that long).

They are suitable for freezing. You can cut into squares, wrap and freeze individually for the best results when thawed.