1. Preheat oven to 350 degrees F. Place butter into 2-quart oval casserole; heat in oven until butter is melted.
2. Combine flour, brown sugar, baking powder and salt in medium-size bowl until blended. Stir in milk to make a batter. Pour batter over melted butter in casserole; do not stir.
3. Combine the strawberries, raspberries, blueberries and granulated sugar in a bowl, tossing to coat. Spoon the berry mixture on top of batter; do not stir.
4. Place aluminum foil on bottom oven rack to catch any drips. Bake pudding in preheated 350 degrees F oven for 1 hour or until top is golden brown. Serve warm or at room temperature.