Preheat oven to 180C/350F and grease a 12 hole muffin tin with butter.
Mix the sourdough starter, pumpkin puree, 2 eggs, vanilla extract, butter and oil in a bowl. Whisk together until well combined. It's ok if the oil sits on the top, it will combine once it is poured into the dry ingredients.
In a separate bowl mix together all purpose flour, salt, baking powder, cinnamon, nutmeg and brown sugar.
Now pour the liquid ingredient mixture into the dry ingredients. Gently fold together until just mixed - don't over mix or the muffins will be tough.
Spoon mixture into greased muffin tin. This recipe will fill a 12 hole muffin (each hole around ¾ full).
Bake at 180C/350F for approximately 20 minutes or until muffins are golden brown.
While the muffins are baking, add the powdered sugar, cinnamon and juice of an orange into a bowl and stir together to form the glaze.
Once the muffins are done, take out of the oven and spoon over the glaze while the muffins are still warm.