Preheat the oven to 375º F. Line a 9"x12" baking sheet with parchment paper and spray or grease the parchment.

In the bowl of a standing mixer, cream together the butter and brown sugar, scraping down the sides as necessary. In a separate bowl, whisk togeher the graham cracker, flour and soda and set aside. In another bowl, whisk together the whole milk, 2 egg yolks and vanilla extract. Alternately, add the dry and the wet ingredients to the creamed butter mixture and beat until fully incorporated.

In the bowl of the standing mixer, combine the egg whites and the sugar and whip until medium peaks form. Fod the egg whites into the graham cracker batter. Pour the batter into the prepared baking sheet and bake for 20 to 23 minutes or until the cake is golden brown and set in the middle.