Cake Base:

1. Preheat the oven to 350°F (175°C). Grease and line an 8-inch round cake pan with parchment paper.
2. In a large bowl, cream together the butter and sugar until light and fluffy.
3. Beat in the egg, followed by the vanilla extract.
4. In a separate bowl, whisk together the flour, baking powder, baking soda, and salt.
5. Gradually add the dry ingredients to the wet mixture, alternating with the milk, beginning and ending with the dry ingredients.
6. Pour the batter into the prepared cake pan and bake for 20-25 minutes or until a toothpick inserted into the center comes out clean.
7. Allow the cake to cool completely in the pan.

White Chocolate Mousse:

1. In a small bowl, sprinkle the gelatin over the cold water and let it sit for 5 minutes to soften.
2. In a saucepan, heat 1/2 cup of heavy cream until it’s just about to boil. Remove from heat and add the softened gelatin, stirring until dissolved.
3. Place the white chocolate chips in a bowl and pour the hot cream mixture over them. Let it sit for a minute, then stir until smooth.
4. In a separate bowl, whip the remaining 2 cups of heavy cream with the vanilla extract until stiff peaks form.
5. Gently fold the white chocolate mixture into the whipped cream until well combined.

Assembly:

1. Once the cake has cooled, remove it from the pan and place it on a serving plate or cake stand.
2. Pour the white chocolate mousse over the cake, spreading it evenly.
3. Refrigerate the cake for at least 4 hours, or overnight, to allow the mousse to set.
4. Garnish with white chocolate shavings or fresh berries before serving.