Preheat the oven to 350 degrees.

Grease a 9x13 baking pan and dust it lightly with cocoa powder. Shake out any excess.

Whisk the flour, sugar, cocoa, powder, baking powder, baking soda, and salt in a large bowl.

Mix in the eggs, milk, oil, and vanilla, beating until smooth.

Whisk in the boiling water until the batter is thinned out and smooth.

Pour the batter into the prepared pan. Bake for 25-30 minutes. A toothpick test should return clean or you can gently press your finger in the center of the cake and it should spring back.

Allow the cake to cool completely in the pan.

Make the topping by melting the chocolate chips in a microwave-safe bowl, cooking in 30-second intervals in the microwave. Stir inbetween each interval until melted and smooth. Allow the chocolate to cool slightly.

Combine the heavy cream, confectioner’s sugar, and vanilla in the bowl of a stand mixer. Beat until soft peaks form.

Fold the melted chocolate into the whipped cream mixture until fully incorporated.

Once the cake has cooled, spread the mousse mixture on top of the cake to cover completely.

Refrigerate the cake for at least 2 hours to allow everything to set before serving.