Preheat the oven to 350ºF

In the bowl of a stand up mixer (or by hand in a medium bowl) beat the butter with the sugar until creamy. With the machine running, add the eggs, one by one.

Dissolve the cocoa in 3 Tbs of hot water and, with the machine running, add to the mix. Sift the flour and baking powder and incorporate to the mix, until a smooth cream forms.

Butter the side and walls of an 8-inch diameter springform and line the bottom with parchment paper. Pour the chocolate mixture, tap on the counter to release any bubbles that may have formed and place in the oven. Bake for 20 minutes, checking at the 15 minute mark. Remove from the oven when the center is still bright and looks slightly undone. Let the cake cool down in the springform for at least 2 hours (you can cover it and place it in the fridge after 1 hour).

In a small saucepan over medium heat, dissolve the sugar in 4 Tbs water. Add the instant coffee and the chocolate chips or semi-sweet chocolate, stir until melted, about 3 minutes.

In a medium bowl, beat the egg yolks. Add the warm mixture and stir to mix.

Whip the cream and add to the bowl with the egg mixture, folding it gently. Pour the mousse over the cake and refrigerate for a minimum of 6 hours. Remove from the fridge when ready to serve.