Preheat the oven to 425°F and generously grease the baking sheet.
In a large bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, and salt.
In a separate bowl, whisk the buttermilk, eggs, melted butter, and vanilla extract until smooth.
Pour the wet ingredients into the dry ingredients and gently fold together until just combined.
Pour the batter onto the prepared sheet pan and spread evenly into the corners.
Add toppings in sections using blueberries, strawberries, or chocolate chips.
Bake for 15 to 18 minutes until golden and set in the center.
Let cool for 5 minutes, then slice into squares and serve warm.