Preheat oven to 350°F (175°C) and line an 8-inch springform pan with parchment paper.
Mix flour, cocoa powder, sugar, baking powder, baking soda, and salt in a bowl.
Add egg, milk, oil, and vanilla extract and mix until smooth.
Stir in hot water and pour batter into prepared pan.
Bake for 25–30 minutes or until a toothpick comes out clean.
Let the cake cool completely inside the pan.
Melt dark chocolate and allow it to cool slightly.
Whip heavy cream with powdered sugar until soft peaks form and fold in melted dark chocolate.
Spread dark chocolate mousse evenly over cooled cake and refrigerate for 30 minutes.
Repeat the same process for milk chocolate mousse and spread over the dark layer, then chill again.
Repeat with white chocolate mousse and smooth the top evenly.
Refrigerate the cake for at least 4 hours or until fully set.
Carefully remove from springform pan, slice, and serve chilled.