Preheat the oven to 425°F. Grease an 18x13-inch half sheet pan with butter, line it with parchment paper, then grease the parchment as well.
In a large bowl, whisk together the flour, sugar, baking powder, baking soda and salt.
In a separate bowl, whisk the buttermilk, melted butter and vanilla. Pour into the dry ingredients and whisk gently until just combined, leaving a few small lumps. Let the batter rest for 5 minutes.
Pour the batter into the prepared pan and spread it evenly into the corners with a spatula. Scatter the blueberries, strawberries and chocolate chips over the top in three sections, so everyone can pick their favorite.
Bake for 15-18 minutes, until the top is light golden, springs back when gently pressed and a toothpick inserted in the center comes out clean.
Brush the hot pancake with the melted butter right away. Let it cool in the pan for 5 minutes, then cut into squares and serve with maple syrup.