Explosion cakes are all the rage in New York City, thanks to Amirah Kassem's bakery Flour Shop. Hers often come with an explosion of rainbow sprinkles, but this version is the sweetest way to tell your family and friends whether you're welcoming a girl or boy.
Preheat the oven to 350 degrees F. Spray three 6-inch cake pans with cooking spray. Line each pan with parchment paper rounds. Spray the parchment paper and flour pans. Tap the pans over a sink or trash can to get rid of excess flour.
Place the cake mix, water, oil and egg whites in the bowl of a stand mixer fitted with a paddle attachment and mix on low speed until moistened, about 30 seconds. Increase the speed to medium for 2 minutes. Dye with food coloring, either a shade of blue or pink depending on the gender. Pour the batter evenly into the prepared pans and bake until a toothpick inserted in the center comes out clean, about 20 minutes. Cool completely on a wire rack.
For the buttercream: Beat the butter in the bowl of a stand mixer fitted with a paddle attachment on high speed until fluffy, about 2 minutes. With the mixer on low speed, gradually add the confectioners' sugar and milk, 1 tablespoon at a time, alternating between the two until no lumps remain. Add the vanilla and salt and mix until incorporated.
Trim the dome off each cake layer. Cut the center out of 2 cake layers with a 2-inch round cookie cutter. Leave the third cake layer whole.
Place 1 cake layer with a hole on a plate or 6-inch cardboard cake drum on a rotating cake stand. Spread some of the buttercream over top with an offset spatula, leaving a 1/2-inch border around the hole. Repeat with the other cake layer with a hole. Fill the center with the blue or pink sprinkles. Spread some of the buttercream over top. Top with the remaining whole cake layer with the bottom facing up. Spread some of the buttercream over top and sides of the cake to create a crumb coat. Chill at least 20 minutes.
Spread the remaining buttercream over the top and sides of the cake.
While the buttercream is still somewhat sticky, hold the cake with one hand over a rimmed baking sheet. With the other hand, grab a handful of the white sprinkles and press up the sides of the cake, starting from the bottom. Cover the top with the white sprinkles. Cut, watch it explode and serve.