Preheat oven to 400F. Grease a 9 x 13 inch baking pan & line base & sides with parchment paper.
Whisk flour, sugar, baking powder, soda & salt together in a medium bowl.
Combine milk & vinegar in a small bowl & let stand 5 minutes. Add half the butter, egg & vanilla; whisk until well combined. Add to flour mixture. Whisk until just combined.
Brush the parchment paper with a little butter. Scrape the batter into the pan. Scatter with the fruit, leaving about 1/2 cup for serving. Drizzle the batter with the remaining butter. Bake for 20 minutes until set. Then broil for 2 minutes, 6 inches away from heat source, until golden. Let stand for 5 minutes before cutting into 6 servings. Serve dusted with icing sugar, maple syrup, whipped cream & extra berries.