Topping: Combine 1 cup of flour, 1 cup of sugar, and 1 teaspoon of baking powder in a small bowl. Beat an egg in a separate bowl. Make a depression in the center of the dry mix and pour the egg into it. Fold gently until the topping mixture is crumbly.
Filling: Mix ¾ cup of sugar and 3 tablespoons of corn starch in a small bowl. Put 6 cups of fresh or frozen blackberries (or boysenberries) in a large bowl and sprinkle with the sugar/corn starch mix. Toss gently to evenly coat the berries.
Grease a square 52 or 58 sqcm pan with butter. Put the berries in it and sprinkle the topping over evenly. Melt a stick of butter and drizzle it evenly over the topping.
Bake this at 191°C for 45 minutes. I predict that unless you utterly disdain dessert, you will have a second helping of this, especially if you serve it with vanilla ice cream . . .