Preheat your oven to 325 F degrees. Grease a springform pan with 1 tablespoon butter, and dust with sugar or cocoa powder. Set aside.
Melt chocolate and butter over simmering water in a double boiler, about 6 to 8 minutes. Remove from heat and set aside to cool slightly
Separate the egg whites and yolks, placing the yolks in a separate bowl and the egg whites in the bowl of a stand mixer fit with the whisk attachment.
In an electric mixer fitted with the whisk attachment, whip yolks, the whole egg, and ¼ cup of the sugar on high speed, until it is pale yellow and falls in a thick ribbon when the beater is raised, about 5 minutes. Add the vanilla, salt, cocoa powder, and espresso powder, beat for 30 seconds or until incorporated.
In an electric mixer, using a clean whisk attachment and bowl, whip egg whites and cream of tartar on medium speed. Gradually add the remaining ½ cup sugar and increase the speed to high. Continue beating on high until medium-stiff peaks form.
While egg whites are beating, add the cooled, melted chocolate slowly to the yolk mixture.
Using a spatula, gently fold half of the egg whites into the chocolate mixture until it begins to incorporate, it will still have a slightly marbled appearance. Add the remaining egg whites. Do not over mix.
Pour the batter into the prepared pan, using the back of a spoon or a rubber spatula to gently spread it into an even layer. Bake for 45 minutes or until a knife inserted into the middle comes out clean and the edges of the cake start to pull away from the sides of the pan.
Allow the cake to cool completely before removing it from the springform pan. Dust with powdered sugar or cocoa powder before serving.
Notes
The top of the cake will fall creating a crackly top. The top should be slightly dry and the interior creamy.
This cake needs to be baked in a springform pan if you want to serve it on a platter. It can't be turned out on to a cooling rack like a standard cake.