1. 1. Preheat your oven to 350°F (175°C). Grease and line two 9-inch round cake pans with parchment paper.
   2. In a medium bowl, whisk together the flour, baking powder, baking soda, cocoa powder, and salt.
   3. In a large bowl, beat together the sugar, eggs, vegetable oil, and vanilla extract until smooth and fluffy.
   4. Gradually add the dry ingredients to the wet ingredients, alternating with the milk. Mix until combined.
   5. Stir in the boiling water (the batter will be thin, but that’s normal).
   6. Divide the batter evenly between the prepared cake pans.
   7. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean.
   8. Let the cakes cool in the pans for 10 minutes, then transfer them to a wire rack to cool completely.

For the Chocolate Mousse:

1. 1. Heat the heavy cream in a saucepan over medium heat until it begins to simmer (but not boil).
   2. Remove from heat and stir in the chopped dark chocolate. Stir until smooth and fully combined.
   3. Let the chocolate mixture cool to room temperature, then refrigerate for about 1 hour, or until it firms up.
   4. Once chilled, whip the mousse with an electric mixer until soft peaks form. Add powdered sugar and vanilla extract and continue whipping until stiff peaks form.