Start by heating your oven to 350°F (180°C). Grease two 8-inch square pans and line the bottoms with parchment paper.
In a large mixing bowl, whisk half of the cocoa powder with half of the boiling water until it’s smooth. Now add half of the sugar, butter, flour, baking powder, salt, eggs, and milk into the same bowl. Use a hand mixer and beat the mixture for about 2 minutes until it looks fluffy and creamy.
Pour the batter into the two pans and smooth the tops with a spatula. Bake for 35 to 40 minutes. Gently press the center—if it springs back, the cakes are ready. Let them cool on a wire rack.
Wash your pans and line them again. Repeat the same mixing and baking steps using the rest of the ingredients to bake two more cakes. Let all four cakes cool completely.
While the cakes are cooling, start the chocolate frosting. In a clean bowl, beat the softened butter with milk and half of the confectioners’ sugar until smooth. In a small bowl, stir the cocoa powder with boiling water to make a rich paste. Add this paste to the frosting, along with the remaining sugar. Mix everything together until it’s thick and fluffy.
Take your first cooled cake and place it on a large cake board or serving tray. Spread a thick layer of frosting on top. Place another cake on top to make the base of your castle.
From the other two cakes, cut out two 4½-inch square pieces using a paper template. Layer them with frosting and stack them in the middle of the base. This forms the top tower of the castle.
Use a 2-inch round cutter to make 10 small circles from leftover cake pieces. Frost and stack them to create 5 mini towers (turrets).
Place one turret on each corner of the bottom layer and one in the center of the top layer. Use a spatula to frost the entire cake and all the turrets until they’re completely covered in chocolate goodness.
Now the fun part—decorating! Stick wafer cookies on the front to make the castle door. Use jelly beans for the doorknobs. Press KitKats or chocolate fingers along the bottom and top layers to look like castle walls.
Fill the tops of the turrets with M&M’s. For the tallest turret, decorate one ice cream cone with jelly beans using edible glue. Set it on the top-center turret. Add the remaining cones upside down on the other turrets to finish the look.
If you have extra decorations like flags, toy soldiers, or mini princesses—go wild! Make your cake as magical as you want.