Preheat the oven to 320 F (160 C). Melt the butter in the microwave or on the stovetop. Set the butter aside to cool.
Crack the egg then open it up and slide the yolk from shell to shell allowing the egg white to fall into one bowl, then place the egg yolk in the other bowl. Do this with all the eggs.
Now using a mixer beat those egg whites, until stiff peaks form. It takes a few minutes 3 to 5 minutes to get it to that stage. Lift the beaters up and if the egg white stays up and doesn't fall over, you are good to go.
Cream together the egg yolks and the sugar using your mixer in a separate large bowl.
Add in the melted butter and mix that in with the yolks and sugar. Switch to a spatula and put the mixer away.
Next, add the flour and cocoa powder to a sifter and add the dry mixture to the batter. Then fold it in with your spatula, just until everything is combined.
Add the milk about 1/3 of it at a time until it is incorporated into the batter. The batter will be fairly runny.
Now we fold in the egg whites with a spatula. Again about 1/3 of the egg whites at a time. It takes a bit of time, just be patient and keep folding until the egg whites are blended in. You may have a few little clumps of egg white. Don't worry about that.
Line an 8x8 or 7x11 inch baking pan with parchment paper. Pour all the batter into the pan. Bake it for 50 to 60 minutes on the middle rack. Check it at 40 minutes. The cake is done when there is just a slight jiggle in the middle of it.
Remove the cake from the oven and allow it to cool completely at room temperature still in the pan. Then place it in the fridge to completely set, 1 to 2 hours.
Once set, remove the cake from the fridge and cut it into squares. Top with powdered sugar and berries if you like. Enjoy!