preheat oven

grease a 9x13 pan

mix the flour and the sugar with the lemon zest, use you hands to rub it in well

cut in the butter until crumbly

remove 3/4 cup of the flour sugar mix for the topping, set aside

add the lemon juice to the buttermilk and allow to rest for a minute

then beat the baking powder, buttermilk and egg yolks into the remaining flour sugar mixture

beat well until thoroughly combined

beat the egg whites until soft peaks form, this can be done quickly with a whisk or with a mixer

fold them into the cake batter

spread the batter into the 9x13 pan

sprinkle with the frozen berries and then reserved flour/sugar/butter mix

bake at 350˚ for 30-35 minutes