Beat the egg yolks with the brown sugar until light golden color. Add butter and vanilla extract and continue beating for another minute or two after which you can add the flour and mix it in until fully incorporated.
Separate eggs and add the egg whites to a mixer and mix until egg whites are stiff. Place egg whites in a bowl and set aside.
Start by heating the butter in a small saucepan over medium heat until melted. Whisk continuously until it becomes golden brown and tiny dark brown flecks appear in the bottom of the pan. Remove from heat, whisk for another 30 seconds and strain through a fine sieve into a bowl. Let cool to room temperature.
Preheat oven to 325 F degrees. Grease a 8 inch x 8 inch baking dish.