Preheat oven to 175C / 350F. Line muffin tins with paper liners. Whisk flour, baking powder and salt in a small bowl until the baking powder and salt are evenly distributed in the flour. Set aside. Whisk cocoa powder and water (or milk) in a separate bowl into a paste. Set aside.
Cream butter, peanut butter and brown sugar on medium-high speed until pale, light and fluffy, scrapping down the sides occasionally, about 5 minutes. Add in one egg, beat on medium speed for 1 minute. Scrap down the sides then add in the second egg and vanilla, and beat on medium speed for 1 minute until incorporated.
With the mixer running on low, beat in the flour mixture until almost incorporated. Use a rubber spatula and finish mixing in the flour mixture. Scoop 1/3 of the cake batter into the cocoa paste and mix gently.
Scoop alternate spoonfuls of cake batter into the muffin tins, filling them about 3/4 full. Swirl the cake batter slightly with a skewer or a toothpick.
Bake for 15 to 18 minutes until a skewer inserted into the center comes out clean. Let cool completely before serving.
NOTES
- Adapted from Cake Keeper Cakes by Lauren Chattman