Preheat oven to 350℉/177℃.
Spray an 8-inch round pan with baking spray and line bottom with parchment paper. Set aside.
Put chocolate pieces and butter in a small heatproof bowl and place over a pot of simmering water.
Stir until melted and smooth.
Remove from heat.
Whisk in cocoa powder.
Set aside to cool.
Beat egg whites until foamy.
While continuing to beat, gradually stream in sugar.
Beat to medium peaks. Set aside.
In a small bowl, whisk vanilla and egg yolks.
Add egg yolk mixture to cooled chocolate mixture and whisk until well combined.
Fold in ¼ of egg whites into chocolate mixture at a time, continuing until no egg whites remain.
Spoon into prepared pan and bake for 12-15 minutes, until cake tests done.