01 - Preheat oven to 350°F. Grease and line three 8-inch round cake pans with parchment paper.
02 - In a medium bowl, whisk together flour, baking powder, baking soda, and salt.
03 - In a large bowl, beat butter and sugar with an electric mixer until light and fluffy, about 3 minutes.
04 - Add eggs one at a time, beating well after each addition. Mix in vanilla extract.
05 - Alternate adding the flour mixture and milk to the butter mixture, starting and ending with flour. Mix until just combined.
06 - Divide batter evenly into five bowls. Tint each bowl with a different neon gel food coloring until the colors are vibrant.
07 - Spoon dollops of each colored batter into the prepared pans. Gently swirl with a skewer or spoon for a marbled pattern.
08 - Bake for 30–35 minutes, or until a toothpick inserted in the center comes out clean. Cool cakes in pans for 10 minutes, then transfer to a wire rack to cool completely.
09 - Beat butter for the frosting until creamy. Gradually add sifted powdered sugar, then mix in heavy cream, vanilla, and salt. Divide into five bowls and tint each with neon food coloring.
10 - Level cooled cake layers as needed. Stack and fill layers with alternating colored buttercream, smoothing between each layer.
11 - Apply a thin layer of buttercream to cover crumbs. Chill cake for 30 minutes.
12 - Frost cake with swirls or stripes of neon buttercream for a vibrant finish.
13 - Heat heavy cream until just steaming. Pour over chocolate chips and let sit for 2 minutes before stirring smooth. Tint with oil-based neon coloring if desired.
14 - Allow drip mixture to cool slightly. Drip around edges of the chilled cake using a spoon or squeeze bottle.
15 - Pipe swirls of remaining buttercream, add sprinkles, or decorate with candy eyes for a festive monster-themed finish.