Preheat oven to 350 degrees. Grease and flour a 13x9x2 inch baking pan. Sift flour with baking powder. In large bowl with clean beaters, beat egg whites until frothy. Add sugar gradually, beating to form stiff peaks. Add yolks, 1 at a time. Slowly add flour and milk.
Pour batter into prepared pan and bake until edges are golden brown, about 40 to 45 minutes. Remove from oven and let cool on a rack.
Topping: Combine milks and sour cream (do not beat). Pour Topping over cake and let sit until all the mixture is absorbed, 20 to 30 minutes.
Prepare meringue and refrigerate: Combine sugar and water in a saucepan. Let sit until sugar is dissolved. In clean electric mixer bowl with clean beaters, beat egg whites. Slowly pour sugar syrup over egg whites, beating constantly until meringue holds stiff peaks. Store in refrigerator.
This can be served immediately but is best made 24 hours ahead and chilled. The meringue will keep, covered, 2 days in the refrigerator.
Before serving, cut cake into squares and spread meringue over each. Garnish as desired with fresh berries and mint leaves.