1. Preheat your oven to 350°F (175°C) and prepare three cake pans by greasing and lining them with parchment paper.
2. Prepare the vanilla cake batter by mixing butter, sugar, eggs, vanilla extract, flour, baking powder, and milk until smooth.
3. Divide the vanilla batter into three equal portions in separate bowls.
4. In one portion, mix in cocoa powder and melted chocolate to create the chocolate batter.
5. In another portion, mix in strawberry puree or strawberry flavoring and a few drops of pink food coloring to create the strawberry batter.
6. Pour each batter into its individual cake pan and spread evenly.
7. Bake all layers for about 20-25 minutes or until a toothpick inserted in the center comes out clean.
8. Let the cakes cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.
9. Prepare the frosting by whipping butter, powdered sugar, vanilla extract, and milk until smooth and fluffy.
10. If desired, divide the frosting into three portions and flavor each to match the cake layers (chocolate, vanilla, and strawberry).
11. Place the chocolate cake layer on a serving plate and spread a thin layer of frosting on top.
12. Add the vanilla cake layer on top of the chocolate layer and spread another layer of frosting.
13. Place the strawberry cake layer on top of the vanilla layer and cover the entire cake with a crumb coat of frosting.
14. Refrigerate the cake for 15-20 minutes to set the crumb coat.
15. Apply a final layer of frosting, smoothing out the sides and top evenly.
16. Decorate as desired with sprinkles, piped frosting, or fruit slices.
17. Slice and serve your Neapolitan cake.