Preheat the oven to 320F.
Coat the bottom of an 8X8 inch baking dish with non stick spray. Line it with parchment paper and coat the paper with non stick spray.
Melt butter in a microwave safe bowl and set aside to cool.
Beat egg whites in a clean and dry bowl until stiff peak. Set aside.
In another bowl, mix egg yolks and sugar until light and creamy.
Add melted butter and mix until fully combined.
Sift all purpose flour and dutch processed cocoa powder into the batter and mix using a spatula until fully combined.
Add milk and mix well.
Fold in the egg whites one third at a time. Fold gently using a spatula and don't over mix.
Pour the batter in to the baking dish and bake for 60 minutes. Check after 40 minutes. The cake is done once the center is jiggles slightly but feels firm.
Let the cake cool in room temperature and then put it in the refrigerator for one to two hours.
Dust with powdered sugar and slice.