Mix egg whites with a pinch of salt on high speed for 4 minutes. Put aside into a different bowl and clean the bowl.
Melt chocolate in a saucepan over low heat. Cut cold butter into saucepan with melted chocolate and continue mixing until blended.
Place the 3 egg yolks with sugar into a mixing bowl and mix together well.
When sugar is dissolved, add butter and chocolate mixture. Keep mixing on low.
Add flour, by spoonfuls on lowest speed.
Add egg white mixture slowly and continue mixing just until fully blended.
Pour batter in to a 12x9 deep dish baking pan.
Prehat oven at 360F and bake for 30 minutes.
Remove and let cool completely.