Sprinkle the gelatin into the cold water and set aside. Beat the egg whites until stiff peaks form.
In a separate bowl, combine the whipping cream and vanilla extract and beat until soft peaks form. Set both bowls aside.
Set a small saucepan to medium heat and add the milk. Just before the milk comes to a boil, turn off the heat and add the gelatin. Once the gelatin has fully dissolved, add the white chocolate and whisk until fully melted. You may need to turn the heat back on to fully melt the chocolate.
Add this mixture to the whipped cream and fold to combine (do not use an electric mixer – it will deflate the filling!). Then add the egg whites and fold to combine, making sure to keep the mixture as airy as possible.
Add a couple drops of blue food colouring and gently fold it a couple times, creating a marble effect.
Pour the mousse into the pan and smooth the surface.
Place the cake onto a large tray and place in the fridge and chill overnight. Because the filling is quite liquid, a little bit may leak out from the edges of the pan. To help this, bunch some paper towel around the base of the pan and keep it on the tray while it’s setting in the fridge.
Run a warm, damp dishcloth around the sides of the pan, then slide a sharp knife along the inside walls of the pan to help loosen the cake. Gently unlatch the sides of the pans and slide the off the cake.
Pour the white candy melts into a shell-shaped chocolate mold and place in the freezer until completely set, about 15 minutes. Use a clean paintbrush to dust the surface with cocoa powder.
Decorate the cake with blue sanding sugar, white sprinkles, pink jelly beans, chocolate shells and pearl-like candies. Slice and enjoy!