Preheat oven to 350°F. Grease a 9" springform pan.
Place butter and chopped semi-sweet chocolate in a large microwave safe bowl. Microwave on Medium until melted, stirring every 30 seconds. Set aside to cool while preparing meringue.
339 g semi-sweet baking chocolate, 200 g unsalted butter
Place egg whites in a clean mixing bowl. Mix on Medium until frothy. Begin adding granulated sugar one spoonful at a time allowing sugar to fully incorporate before adding next scoop.
8 large eggs, 198 g granulated sugar
Once the sugar is added, turn mixer to Medium-High and whip until stiff peaks form. Set aside.
Whisk egg yolks into chocolate mixture. Add cocoa powder, salt, coffee and vanilla.
8 large eggs, 42 g unsweetened cocoa powder, 1/2 teaspoon salt, 1 tablespoon strong coffee or espresso, 1 teaspoon vanilla extract
Using rubber spatula, add 1/4 of meringue to chocolate mixture, stirring gently, to loosen the batter.
Continue adding 1/4 of the meringue at a time to chocolate mixture, folding gently to retain as many air bubbles as possible.
Once meringue is fully incorporated, carefully pour batter into prepared pan.
Bake for 40-45 minutes until the center is set.
Set aside to cool, cake will fall and crack.
Serve with powdered sugar, cocoa powder, whipped cream or ice cream if desired.