Preheat oven to 350°F (175°C)
Grease and line an 8-inch round cake pan
Mix flour, cocoa powder, sugar, baking powder, baking soda, and salt
Add egg, milk, oil, vanilla, and hot water; mix until smooth
Bake for 22–25 minutes
Cool completely
Whip heavy cream with powdered sugar and vanilla until soft peaks form
Gently fold in melted chocolate until smooth and airy
Place cake on a serving plate or inside a springform ring
Spread mousse evenly over the cake layer
Smooth the top with a spatula
Refrigerate for 3–4 hours, or until fully set
Remove ring, decorate as desired, slice, and enjoy