Melt the chocolate and butter together over the stove or in the microwave. Set in the fridge to cool.
While the chocolate/butter mixture is cooling, separate the eggs. Put the egg yolks in a small bowl and the egg whites in the bowl of a stand mixer (or a medium bowl if using a handheld mixer).
Beat the egg whites until foamy. Then, slowly stream in the brown sugar while turning the mixer to high speed. Beat for 5-7 minutes until stiff peaks form.
While the egg whites are beating, add the yolks to your chocolate mixture one by one, whisking throughly between each addition.
Add the cocoa powder, salt and vanilla and fold together until combined.
Once the egg whites are done, add spoonfuls of the egg whites into your batter, folding it together in between each spoonful. Repeat this process until all of the egg whites are incorporated into the batter.
Gently pour the batter into a greased 9x9 or 8x8 inch cake/pie pan. Bake for 45-55 minutes at 325 degrees Fahrenheit until a toothpick comes out clean. Enjoy!