Place the flour, cocoa powder, sugar, baking soda, baking powder and salt in the bowl of an electric mixer and mix on low speed until fully combined. Add the egg, milk, oil, vanilla and water and mix until smooth. Line the sides and bottom of a 9-inch springform pan with parchment paper and pour the batter into the pan. Bake at 350F for 20-30 minutes, or until a skewer inserted into the center comes out clean. Place the pan on a cooling rack and cool completely in the pan. Remove the cake from the pan and remove the parchment paper lining. Return the cake to the pan and set aside.
Make the milk chocolate mousse:
Sprinkle the gelatin into the cold water and set aside.
Beat the egg whites until stiff peaks form. In a separate bowl, beat the whipping cream until soft peaks form and 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 milk 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 whisk to combine. Then add the egg whites and whisk to combine, making sure to keep the mixture as airy as possible. Pour the mousse on top of the chocolate cake and smooth the surface. Transfer the cake to the fridge to set for 2 hours.
Make the white chocolate mousse:
Follow the same steps for the Milk Chocolate Mousse, but replace the milk chocolate with white chocolate. Once you have gently poured the white chocolate mousse onto the cake, transfer the cake to the fridge and set for 2 ½ hours.
Place the paper snowflake cutouts onto the surface of the cake. To help with positioning, use some scotch tape to create little handles on one side. Sift cocoa powder evenly over the entire surface of the cake. Gently remove the cutouts to reveal white snowflakes! Gently unlatch the sides of the pan and slide it off the cake. Running a warm, damp dishcloth around the sides of the pan will make it easier to remove. To serve, slice the cake with a warm knife.