Instructions:

Cake Preparation:

Preheat Oven:
Preheat your oven to 350°F (175°C). Grease and flour an 8-inch cake pan.

Mix Dry Ingredients:
In a large bowl, whisk together the flour, sugar, cocoa powder, baking powder, baking soda, and salt.

Add Wet Ingredients:
Add the egg, milk, oil, and vanilla extract; beat until well combined.

Add Boiling Water:
Stir in the boiling water until the mixture is smooth. The batter will be thin.

Bake the Cake:
Pour the batter into the prepared cake pan and bake for 30-35 minutes, or until a toothpick inserted into the center comes out clean.

Cool the Cake:
Allow the cake to cool in the pan for 10 minutes, then transfer to a wire rack to cool completely.

Chocolate Mousse Preparation:

Melt Chocolate:
In a microwave-safe bowl, combine the chocolate chips and 1/4 cup of heavy cream. Microwave in 30-second intervals, stirring in between, until melted and smooth.

Whip Cream:
In a separate bowl, beat the remaining heavy cream with sugar and vanilla extract until stiff peaks form.

Combine:
Fold the melted chocolate into the whipped cream until no streaks remain.

Chill Mousse:
Refrigerate until set, about 1 hour.
Assembly:
Slice Cake:

Slice the cooled cake horizontally into two layers.
Layer Cake:

Place the bottom layer on a serving plate and spread half of the chocolate mousse over the top.

Add Second Layer:
Place the second layer of cake on top and spread the remaining mousse over the top and sides of the cake.