Preheat oven to 350°F. Grease one 9" cake pan and line the bottom with parchment paper.

Toss chopped peanut butter cups with 1 Tbsp of flour, set aside.

In a mixing bowl, cream butter, peanut butter, and sugar until light and fluffy. Beat in eggs, 1 at a time.

In a separate bowl, sift together the flour, baking soda, and salt. At low speed, add the flour mixture alternately with the buttermilk.

Add the chopped peanut butter cups to the batter. With a spatula, mix until combined.

Spoon batter into the prepared pan and spread evenly. Bake cake for 33-35 minutes or until a toothpick inserted in the middle comes out clean. Cool cake for 5 minutes. Turn out onto a rack, peel off parchment. Cool cake completely before frosting.

To make the frosting, in a mixing bowl add the butter, peanut butter, and 1/3 cup heavy cream. Stir at low speed until combined. Slowly add the powdered sugar, 1/2 cup at a time, mixing well after each addition. When all the sugar is added, increase the speed and mix for 2 more minutes. If needed, slowly add a few more drops of heavy cream.

Frost the cake completely. Decorate with peanut butter cups.

Keep cake covered. Serve at room temperature.