1. Preheat oven to 350ยฐF (175ยฐC). Grease and flour three 9-inch round cake pans.
2. In a large bowl, combine flour, sugar, cocoa powder, baking powder, baking soda, and salt. Add eggs, milk, oil, and vanilla extract. Beat on medium speed for 2 minutes. Stir in boiling water until well combined. Fold in chopped peanut butter cups.
3. Divide the batter evenly among the prepared pans. Bake for 30-35 minutes or until a toothpick inserted in the center comes out clean. Allow the cakes to cool in the pans for 10 minutes, then transfer to wire racks to cool completely.
4. For the frosting: In a mixing bowl, cream butter and peanut butter until smooth. Gradually add powdered sugar, alternating with heavy cream. Mix in vanilla extract until creamy and smooth.
5. Level the cakes if needed, then stack and frost them with the peanut butter frosting. Chill the cake for 30 minutes.
6. Divide the remaining frosting into three bowls. Tint one bowl with blue food coloring, one with purple, and one with black.
7. Using an offset spatula, spread the colored frosting randomly on the cake to create a galaxy effect.
8. Drizzle chocolate ganache over the top of the cake and let it drip down the sides.
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