Preheat the oven to 350°F (175°C) and prepare a 9-inch round cake pan by greasing it lightly and lining the bottom with parchment paper for easy release.

Cream the unsalted butter and granulated sugar together in a mixing bowl until light and fluffy, which will take about 3-5 minutes.

Add the room temperature egg and egg yolk to the butter mixture, mixing until fully combined and smooth.

Whisk the all-purpose flour, cocoa powder, baking soda, and salt in another bowl.

Combine the wet and dry ingredients gradually, alternating with the milk and white vinegar while mixing.

Stir in the red food coloring until you achieve a consistent and vibrant shade of red.

Pour the batter into the prepared baking pan and bake for 25-30 minutes, or until a toothpick inserted in the center comes out clean.

Soak the gelatine sheets in cold water until softened while the cake is baking.

Whip 120 g of heavy cream in a separate bowl until soft peaks form.

Mix the cream cheese and granulated sugar together until completely smooth in another bowl.

Heat the lemon juice gently and dissolve the softened gelatine sheets in it.

Fold in the whipped cream gently into the cream cheese mixture until well combined.

Layer the mousse onto the cooled red velvet sponge once baked, spreading it evenly.