Preheat your oven: Begin by preheating your oven to 350°F (175°C) and greasing a half-sheet baking pan.
Mix dry ingredients: In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt until combined.
Add wet ingredients: Add the eggs, milk, vegetable oil, and vanilla extract. Beat on medium speed for about 2 minutes, then stir in the boiling water (the batter will be thin).
Pour and bake: Pour the batter into the prepared baking pan and bake for 30 to 35 minutes or until a toothpick inserted in the center comes out clean.
Prepare peanut butter frosting: While the cake cools, beat together the creamy peanut butter, softened butter, and powdered sugar until smooth and creamy.
Frost the cake: Once the cake is completely cooled, spread the peanut butter frosting evenly across the top and sprinkle with chopped peanut butter cups.
Serve and enjoy: Cut into squares and serve. Enjoy this indulgent dessert with friends and family!