1.Preheat oven to 350 degrees. Grease an 8×8 or 9×9 glass square pan, set aside.
2.In a medium bowl, whisk together the flour, baking soda, and salt. Set aside.
In a large bowl, cream together the butter and sugars until creamy and smooth. Beat in the egg, then stir in sour cream or yogurt with vanilla extract. Mix in the mashed bananas.
3.Slowly add the dry ingredients and mix until flour is well incorporated. Toss the peanut butter cups and gently mix in. Spread cake batter evenly into the prepared pan.
4.Bake for 20 to 25 minutes, or until a toothpick inserted into the center comes out clean or with moist to dry crumbs. Allow cake to cool completely on wire rack.
5.While the cake is cooling, make the peanut butter frosting. In the bowl of a stand mixer, or using a hand held mixer, beat the butter and peanut butter on high until smooth. Slowly add the powdered sugar, Beat on high until powdered sugar is incorporated. Add the milk and vanilla extract. Beat until frosting is smooth.
6.Frost the cake then sprinkle the crumbled peanut butter cups on top. Store on counter for one day or in fridge up to 3 days.
Enjoy the milk chocolate peanuty deliciousness!