Preheat oven to 350°. Line your muffin tins with paper liners.
Add cocoa, flour, baking powder, and salt to a medium bowl and whisk to combine. Set aside.
In an electric mixer fitted with a paddle attachment, cream the butter and sugar until light and fluffy. Add your eggs, one at a time beating well in between each addition. Beat in the vanilla. Add half the flour mixture on low speed along with half the sour cream. Mix until just combined. Repeat with remaining flour mixture and sour cream. Fold in the peanut butter Oreos.
Scoop batter into muffin liners, filling each about ¾ full. Bake about 20 minutes, until a toothpick inserted in the middle comes out clean. Cool completely before frosting.