Preheat oven to 400 and grease a 12 X 18 X 2 cake pan with butter and dust with flour. The pan should be the size of a half sheet pan, but higher. There are commercial inserts for sheet pans to make them into cake pans that are perfect for this use.
Toss the fruit with the 45 grams of flour
Cream the butter, sugar, kirsch and vanilla (you can substitute Brandy or Cognac for the Kirsch) till light and fluffy.
Add the egg and stir in.
Mix the whole wheat flour, salt and baking powder. NOTE: use a baking powder that does NOT contain aluminum or your end result will taste bitter.
Add flour mixture and milk to the batter, alternating between them to make about 3 additions. First mix in the flour.
Grease a full sized sheet pan
Turn the batter out into the greased and floured cake pan. Spread flat.
Distribute fruit across top of batter. Bake for about 30 minutes or till golden.