Preheat oven to 350°F (175°C) and grease a 9-inch springform pan. Whisk together dry ingredients: flour, sugar, cocoa, baking soda, baking powder, and salt in a large bowl.

Add eggs, milk, oil, and vanilla to the dry ingredients, mixing until just combined. Stir in hot water until the batter is smooth, then pour into the prepared pan and bake for 30-35 minutes until a toothpick inserted comes out clean. Cool completely.

Sprinkle gelatin over cold water and let bloom for 5 minutes. Melt milk chocolate in a heatproof bowl over simmering water or in the microwave; let cool slightly.

In a saucepan, heat 0.5 cup of heavy cream and sugar until the sugar dissolves. Stir in bloomed gelatin until fully dissolved. Combine with melted chocolate and the remaining 1.5 cups of cold heavy cream, then whip until soft peaks form and fold into chocolate mixture. Pour over the cooled cake base and chill for at least 1 hour.

Repeat the process for the milk chocolate mousse using dark chocolate, preparing gelatin and combining with heated cream and sugar. Whip the remaining heavy cream until soft peaks form and fold into the dark chocolate mixture.

Carefully spread the dark chocolate mousse over the chilled milk chocolate mousse layer. Refrigerate the cake for at least 4 hours, or preferably overnight, until firm.

Once firm, remove the cake from the springform pan. Garnish with chocolate shavings or cocoa powder before serving.