For the Cake: Preheat the oven to 350°F (175°C). Grease and flour an 8-inch round cake pan and line the bottom with parchment paper.

1. In a medium bowl, whisk together the flour, cocoa powder, baking powder, baking soda, and salt. Set aside.
2. In a large bowl, cream the softened butter and sugar together until light and fluffy, about 2-3 minutes.
3. Add the eggs, one at a time, beating well after each addition. Stir in the vanilla extract.
4. Gradually add the dry ingredients to the wet ingredients, alternating with the milk, and mix until just combined.
5. Carefully pour in the boiling water, mixing until the batter is smooth. The batter will be thin, but this is normal.
6. 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.
7. Let the cake cool in the pan for 10 minutes, then remove it from the pan and transfer it to a wire rack to cool completely.
8. For the Mousse: Place the chopped semi-sweet chocolate in a heatproof bowl. Heat the heavy cream in a saucepan over medium heat until it just begins to simmer.
9. Pour the hot cream over the chocolate and let it sit for 2-3 minutes to melt. Stir until the mixture is smooth and glossy. If using instant coffee, add it here and stir until dissolved.
10. Let the chocolate mixture cool to room temperature.
11. Once the chocolate mixture has cooled, whip the remaining 1 cup of heavy cream with powdered sugar and vanilla extract until stiff peaks form.
12. Gently fold the whipped cream into the cooled chocolate mixture until well combined and fluffy.
13. Assembling the Cake: Once the cake has completely cooled, slice it in half horizontally to create two layers.
14. Place the bottom layer of the cake on a serving platter. Spread a generous layer of chocolate mousse on top.
15. Place the second layer of cake on top and spread the remaining mousse over the top and sides of the cake.
16. Chill the cake in the refrigerator for at least 2 hours to allow the mousse to set.
17. For the Ganache (optional): Heat the heavy cream in a small saucepan until it begins to simmer. Pour the hot cream over the chopped chocolate and let it sit for 2-3 minutes.
18. Stir until the ganache is smooth and glossy.
19. Once the ganache has cooled slightly, pour it over the top of the chilled cake and spread it evenly with a spatula.