1. Preheat oven to 350 degrees F. Butter a 9-by-9-inch cake pan.
2. In a small saucepan over medium heat, melt 4 tbsp butter and the brown sugar. Pour the mixture into the bottom of the cake pan. Place the cranberries in the cake pan on top of the melted butter and sugar.
3. In a small mixing bowl, using clean beaters, beat the egg whites to soft peaks. Add the cream of tartar and continue to beat until the peaks hold their shape. Set aside.
4. Mix together the flour, baking powder, and salt. In a mixing bowl, cream the remaining 1/2 cup butter with the sugar. Add the egg yolks, one at a time, beating well after each addition. Add 1 tsp vanilla and mix well. Add the milk, alternately with the dry ingredients, folding well after each addition.
5. Fold the whites into the cake batter. Spread the batter over the fruit in the cake pan, and bake until a toothpick inserted into the center of the cake comes out clean, about 55 to 60 minutes.
6. Remove the cake from the oven, and run a small knife around the edges. Let sit for 15 minutes. Invert the cake onto a serving platter and let it sit another 5 minutes, then remove the pan.
Optional: Whip heavy cream to soft peaks. Flavor with the remaining vanilla and the confectioners sugar.