Preheat the oven to 425°F. Brush the pan and sides with butter.
Whisk the flour, sugar, baking powder, and salt in a large bowl until well combined.
In a separate bowl, whisk the milk, eggs, melted butter, vanilla, and lemon zest until smooth.
Pour the wet ingredients into the dry and whisk just until no dry pockets remain. Avoid overmixing.
Pour the batter into the prepared pan and spread into an even layer. Tap the pan on the counter to level.
Scatter the strawberries, blueberries, raspberries, and chocolate chips evenly over the surface, or divide the pan into sections and add a different topping to each.
Bake for 15 to 18 minutes, until the top is set and lightly golden and the center springs back when touched.
Cool for a couple of minutes, slice into squares, and serve warm with maple syrup or fruit sauce.