Preheat the oven to 350°F. Set aside an 8 or 9-inch round springform pan (do not grease the pan).
Melt the chocolate, butter, and instant coffee granules in a microwave-safe bowl on half power, stirring halfway through, until smooth. All microwaves are different, so this can take anywhere from 1-2 minutes. Alternatively, you can melt the chocolate and butter over a double boiler.
Let the chocolate mixture cool to barely lukewarm and then add two whole eggs, stirrring after each addition. Separate 4 eggs, placing the whites into the bowl of a stand mixer and the yolks into the chocolate mixture. Stir the yolks into the chocolate mixture, followed by ½ cup (100 g) of the granulated sugar.
Whip the egg whites until frothy, then gradually add the remaining ½ cup (100 g) of granulated sugar and beat until glossy soft peaks form, about 5 minutes. They will hold their shape, but not stand straight up and be stiff.
Gently fold about ¼ of the whites into the chocolate mixture to lighten it. Then carefully fold the remaining whites into the chocolate until no whites are seen. Pour the batter into the pan and smooth the top.
Place the springform pan on a baking sheet and bake the cake until the top looks puffed and the center is no longer jiggly, about 35 to 40 minutes. Don't allow it to bake beyond this point.
Let the cake cool in the pan on a wireless cooling rack. It will sink in the center as it cools, which is perfectly normal and a good thing!