Preheat oven to 350°F. Grease an 8”x8” baking pan and line it with parchment paper (allow the paper to overlap the edges by an inch or two to make it easy to lift out the squares later). Separate 2 eggs. Put whites in the fridge until needed.
In a large bowl, cream butter and sugar. Beat in egg yolks.
In another bowl, combine flour, baking powder and salt. Add dry ingredients to yolk mixture. Blend in milk and vanilla to make a smooth cake batter. Spoon mixture into prepared baking pan. Bake for 15-20 minutes, until cake tester comes out clean.
While cake is baking, prepare meringue topping. Beat egg whites until firm and standing in peaks. Beat in sugar and salt. Gently fold in berries with a spatula.
Spoon meringue over top of baked cake and return to oven until meringue is lightly browned (10-15 minutes). Cool completely before cutting. Grip the parchment paper edges to carefully life the entire square out of the pan and place it on a cutting board. Use a large knife to cut out squares to desired size. (Note: Our meringue had a blue tinge because we used blueberries that were thawed from frozen; for a nice white meringue, use fresh berries.)
Makes 16-20 squares.