Make the cakes: Preheat the oven to 350°F. Spray three 7-inch pans (or 8-inch pans) with flour-based baking spray.
In the bowl of a standing mixer fitted with the paddle attachment, cream the butter and sugar together. Add the eggs and vanilla and beat until combined. Combine the flour, baking powder and salt and add to the mixer bowl in three batches, alternating with the milk and beginning and ending with the flour mixture. Scrape down the bowl and beat on low speed for 3 minutes. Divide the batter evenly into pans, about 2 cups of batter per pan. Bake for 25-30 minutes or until a toothpick comes out clean when inserted into the middle of the cakes. Let the cakes cool slightly in the pans. Remove and let them cool completely on a wire rack. Level the cakes if needed.
Make the custard: In a medium saucepan mix the sugar and flour together. Beat in the whole egg and yolks. Add the milk. Mix well. Whisk in the pinch of salt and vanilla bean seeds. Place over medium-high heat and whisk while cooking until the mixture begins to boil and thickens just like pudding. Remove from the heat and add the butter. Mix well until the butter is melted. Strain the mixture through a sieve into a shallow dish and let cool slightly. Place a sheet of plastic wrap directly on the custard so it doesn’t form a skin. Chill in the refrigerator, about 1 hour. When custard is chilled, it should be firm enough to hold in a spoon (see blog post for example picture).
Make the frosting: In a stand mixer fitted with the whisk attachment, mix together the butter and confectioners’ sugar. Begin mixing on low speed until the mixture is crumbly, add cream a little at a time until the mixture just comes together and increase to high speed. Add vanilla extract. Beat for 3 minutes. Add more cream as needed until the mixture is piping consistency.
Assemble: Remove 1 1/2 cups of the frosting and place in a piping bag fitted with a 1/2-inch open tip. Use 3 drops of sky blue food color and 1 drop neon green gel food color to tint the remaining frosting. Transfer the blue-green frosting to a piping bag fitted with a 1/2-inch open tip. Pipe a ring of frosting around one top edge of a cake. Fill the center with chilled pastry cream. Stack a second cake on top and repeat the process; top with the remaining cake.
Frost the cake with the remaining blue-green icing. Smooth the frosting using a offset spatula that has been warmed (dip in warm water, wipe clean, then smooth across the frosting). Attach egg decors to the sides of the cake, if using.
Pipe dollops of frosting all over the top of the cake and sprinkle on white nonpareils. Add birthday candles if appropriate.
Serve to delighted friends and family.
Keep cake chilled in the refrigerator. Bring to room temperature before serving.