Preheat the oven to 350°F / 170°C. Line a 9" spring form pan with parchment paper and lightly grease the paper and sides of the pan. Set aside.
Add an inch of water to a saucepan and bring to a simmer over medium high heat. Turn the heat down so that the water lightly simmers. In a heat-proof bowl combine the chopped chocolate and unsalted butter. Place the bowl over the simmering water. Let the butter and chocolate melt together, stirring occasionally.
When the chocolate and butter are fully melted add the vanilla, instant coffee powder, salt and 6 tbsp of the sugar. Whisk together and leave on the heat for another minute or two to let the sugar dissolve.
Remove from the heat and whisk in the cocoa powder until fully incorporated. Add the 6 egg yolks, 1 or 2 at a time, to the chocolate mixture, whisking constantly. Ensure the egg yolks are fully encorporated and then set aside to make the meringue.
Add the 6 egg whites to the bowl of a mixer and beat with a whip attachment. When the egg whites become foamy begin adding the remaining 6 tbsp of sugar gradually, a little bit at a time. Beat until the meringue has stiff peaks.
Fold the egg whites into the chocolate mixture. Add a quarter of the egg whites to the chocolate and whisk together to lighten the batter. Add the remaining egg whites in three batches, gently folding them in with a spatula. Continue folding until all the egg whites are incorporated being careful not to overmix and deflate the batter.
Pour the batter into the prepared springform pan. Bake in the preheated oven for 35 minutes or until done. The cake will look puffed and should wobble slightly when shaken. A toothpick inserted into the centre of the cake should have some moist crumbs attached. If the toothpick comes out with some batter on it then bake for another 5 minutes or so.
Let the cake cool in the springform pan for about 15 minutes and then remove the sides. Let cool to room temperature. This cake is easier to cut once it has cooled down. Serve with lots of whipped cream!