Begin by preheating the oven to 350° F (180° C) and line a muffin tin with 6 paper liners.
Whisk the egg in a large bowl until combined. Add the remaining ingredients except for the blueberries and stir together.
Fold in the blueberries gently and divide the batter between the 6 lined muffin tin wells.
Bake the pupcakes for 18-20 minutes, until lightly golden brown. Test with a toothpick and if it comes out clean, they are ready.
Cool in the pan for about 5 minutes then remove the pupcakes to a cooling rack to cool completely. Decorate the tops with additional peanut butter or leave plain.