Preheat your oven to 350°F (175°C) and grease a 9x13-inch baking pan.
In a large mixing bowl, whisk together the flour, granulated sugar, baking powder, baking soda, and salt.
In another bowl, mix the peanut butter, whole milk, vegetable oil, eggs, and vanilla extract until smooth.
Gradually add the wet ingredients to the dry ingredients, stirring until just combined.
Fold in the chocolate chips and chopped Reese’s Peanut Butter Cups, then pour the batter into the prepared baking pan.
Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean.
While the cake is baking, prepare the frosting by mixing the powdered sugar, cocoa powder, and milk until smooth.
Once the cake has cooled for about 10 minutes, drizzle the frosting over the top and let it set before serving.