Preheat your oven to 350 degrees F (175 degrees C) and grease mini (3 inch) springform pans.
In a large bowl, beat the eggs and sugar until the sugar is dissolved and the mixture is fluffy. Mix in the oil, milk, baking powder, salt, and vanilla paste, and combine thoroughly. Divide your cake batter into two equal portions, add the mashed bananas to one portion and the cacao (or cocoa) powder to the other.
Spoon the two different batters into each mini springform pan, alternating one then the other until ⅔ full. Bake at 350 degrees F (175 degrees C) for 25 minutes, or until an inserted toothpick or knife comes out clean. Cool in the pans for 10-15 minutes, then open the rings and remove the cakes. Transfer to a wire cooling rack to cool completely.
Peanut Butter Frosting
In a medium bowl, or the bowl of your stand mixer, combine the softened butter and creamy peanut butter until smooth. Gradually add in the confectioners sugar as well as the heavy cream, 1 Tbsp at a time, until all of the confectioners sugar has been incorporated. Mix in the vanilla paste (or extract).
Assemble the Cakes
Cut the domed top off of the mini cakes, then slice each cake in half. Pipe the peanut butter frosting into the middle of each cake. Mix your (thick) chocolate ganache by combining 12 oz chocolate chips and ½ cup (1 stick) butter and microwaving in 30 second intervals until melted and smooth. Allow to cool slightly before spreading evenly on the tops of the mini cakes.
Let the chocolate finish cooling before piping more of the peanut butter frosting onto the middle top of each mini cake. Unwrap mini Reese's Peanut Butter Cups and place them in the center of the peanut butter frosting on the top of each mini cake.