Preheat your oven to 350°F (175°C). Line an 8-inch square baking pan with parchment paper. In a medium bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt.

In a stand mixer, beat the softened butter, creamy peanut butter, granulated sugar, and brown sugar on medium-high speed for about 3 minutes until light and fluffy.

Add the eggs one at a time, followed by the vanilla. On low speed, alternate adding the dry flour mixture and the buttermilk, beginning and ending with the flour. Mix just until combined.

Pour the batter into the prepared pan and bake for 30 to 35 minutes until a golden-brown crust forms and a toothpick comes out clean. Let cool completely.

Beat the remaining butter and peanut butter until smooth. Gradually add powdered sugar, heavy cream, and vanilla. Whip on high for 2-3 minutes until thick and creamy.

Spread the frosting over the cooled cake using an offset spatula to create decorative swirls. Garnish with roughly chopped roasted peanuts before slicing.