Preheat oven to 180°C. Grease a 12 hole x 1/2-cup capacity muffin pan or line with patty cases.
Sift flour and baking powder into a large bowl. Stir in sugar and choc-bits.
Whisk buttermilk or milk, eggs, vanilla and melted butter in a large jug until well combined. Using a large metal spoon, stir mixture into dry ingredients until just combined. Gently swirl through frozen Mixed Berries (do not over-mix).
Spoon mixture into prepared muffin pan. Bake for 25-30 minutes or until cooked when tested with a skewer. Stand in pans for 5 minutes before turning out onto a wire rack to cool. Lightly dust with icing sugar if desired.