For the Peanut Butter Cake:
1. Preheat the oven to 350°F.
2. Grease and flour three 9-inch round cake pans. Set aside.
3. In a medium bowl, whisk together the flour, baking powder, baking soda, and salt.
4. Using an electric mixer, beat the JUSTIN’S® Classic Peanut Butter, butter, and sugars together on medium speed until light and fluffy.
5. Add the eggs one at a time, beating for 1 minute between additions and scraping down the sides of the bowl before adding the next.
6. Reduce the mixer speed to low and alternate adding ½ cup of the flour mixture and ⅓ cup of the buttermilk until just combined.
7. Divide the batter between the prepared pans and bake until a toothpick inserted into the center comes out clean, for 15 to 22 minutes.
8. Cool in the pans for 10 minutes, then cool completely on a wire rack before frosting.
For the Chocolate Buttercream:
1. For the chocolate buttercream, in the bowl of an electric mixer, cream together butter and cocoa powder.
2. Alternate adding the sugar and milk, beating on high speed after each addition. Add vanilla and mix well.