Reese's Peanut Butter Cup Cake is filled with peanut butter cups and a rich filling of homemade peanut butter frosting. This easy, irresistible treat is perfect for a party or potluck!
In a large bowl, mix together cake mix, eggs, 1 cup milk, vanilla extract, 1 cup of peanut butter, and 1 stick of butter. Spread mixture in a greased 9x13" baking dish.
With a hand mixer, blend 1 stick of butter, 1 cup of peanut butter, and ¼ cup milk. Slowly add in powdered sugar until the frosting is mixed well.
Spoon half of the peanut butter frosting mixture on top of the cake batter. Add 6-8 large squirts of chocolate syrup. Very gently swirl frosting and syrup into the cake mix. Don't completely mix. Make sure you get swirls into the edge of the cake pan.
Gently press in half of the chopped peanut butter cups.
Bake the cake for 35-40 minutes at 350 degrees. Allow cake to cool for 15-20 minutes.
Top the cake with the remaining frosting, more chocolate syrup, and the remaining chopped peanut butter cups.
Notes
I think this cake is best fresh out of the oven. Give it 15-20 minutes to cool, frost/top it, then serve ASAP - it's still warm and the chocolate and peanut butter are perfectly melty!
Vanilla ice cream is incredibly yummy with this warm cake.
Sometimes, you might be able to find the really small peanut butter cups that come unwrapped. You can definitely use those instead of the miniature peanut butter cups...no chopping needed!
I recommend storing this one in the fridge in an airtight container. There's milk in the icing...you don't want that to spoil! It's best within 2-3 days of baking.
This cake is best at room temperature or even a little warm, so try to take your cake out of the fridge 30 minutes to an hour before serving. You can pop pieces of the cake in the microwave to reheat - just set the microwave to about 15 seconds.