Prep: Begin by preheating the oven to 350 degrees F. Grease the inside of a 10-cup bundt cake pan and set it aside.
Batter: In a large mixing bowl, combine the cake mix, brownie mix, eggs, water, and oil. Stir to combine. Pour half of the batter into the prepared bundt cake pan, set the rest aside.
Cream cheese filling: In a separate bowl, combine the cream cheese, sugar, egg, and peanut butter and mix using an electric mixer, or whisk, until the mixture is smooth.
Drop spoonful's of the peanut butter mixture on top of the bottom layer of batter. Then top the peanut butter mixture with the rest of the prepared batter.
Bake: Place the bundt cake pan onto a baking sheet and bake for 50-60 minutes or until a toothpick can be inserted in the center and comes out clean. Do not skip this step as the cake can overflow as it bakes, and the baking sheet will catch any of the mess created.
Cool the cake for 10 minutes before inverting onto a wire rack to cool completely.
Make ganache: Place the heavy whipping cream into a bowl and microwaving for 45 seconds. You want the cream to be nice and hot, add another 30 seconds if needed.
Add the chocolate chips to the bowl and then place a plate on top of the bowl and allow it to rest and sit for 5 minutes undisturbed. Gently whisk the chocolate and cream together until it’s melted and creamy.
Decorate: Drizzle the ganache over the top of the cake using a fork or spoon.
In another small bowl, melt the peanut butter for 20 seconds, and then drizzle it over the top of the ganache layer.
Top the cake with whole, or chopped, mini-Reese’s cups if desired. Slice, serve and enjoy this delightful cake!