For the cupcakes:

1. Combine flour, cocoa powder, baking soda, and salt in a medium-sized mixing bowl and whisk to combine. Set aside.
2. Cream butter, sugars, and peanut butter until smooth.
3. Into the creamed mixture, add one egg at a time, waiting to add the second until the first is fully incorporated.
4. Add 1 cup of milk and the vanilla and mix on low speed until incorporated. Batter will be liquidy at this point.
5. Add dry mixture to the wet ingredients in thirds, mixing on low speed as you add the dry ingredients. When the mixer starts to resist, bump up to medium speed and continue to combine, scraping the sides of the bowl as you go. (I also added that extra 1 tablespoon of milk here to moisten up the mixture a bit as it combined.) Batter will be thick, but light and creamy.
6. Fold in chocolate chips.
7. Line (24) cupcake tins with wrappers and fill 2/3 of the way full with batter.
8. Bake at 350 degrees F for 18-20 minutes, or until a toothpick inserted in the center of the cupcake comes out clean.
9. Cool the tray on a wire rack for 5 minutes, then remove cupcakes from tin and cool completely on wire racks.

For the buttercream:

1. Cream butter and peanut butter together until smooth.
2. Stir in 1 cup of powdered sugar.
3. Add vanilla extract, then 2 Tablespoons of milk. Mix on low speed until incorporated.
4. Add an additional 1 cup of powdered sugar and 1 Tablespoon of milk. Continue to mix.
5. Continue adding powdered sugar and milk until you attain desired consistency–if frosting is too thin, add more powdered sugar. If frosting is too thick, add more milk. (I found that the peanut butter made this frosting pretty thick to begin with, so I didn’t need as much powdered sugar as I initially anticipated.)
6. Frost the cooled cupcakes and garnish with brown sugar if desired!