Using a sheet pan as a vehicle to makes simple baked goods, like muffins (brownie batter and cake batter work well, too), is brilliant. It takes less time to bake, keeps the muffins moist and delicate, and gives you extra browned edges. This technique basically yields a giant tray of muffin tops, so if that's your favorite part of a muffin, you're in luck! —Posie (Harwood) Brien
Preheat your oven to 375°F. Line a baking sheet with parchment paper.
In a large bowl, whisk together the flour, sugar, salt, baking powder, and cinnamon (if using).
In a separate bowl, whisk together the butter, eggs, milk, and vanilla.
Add the wet ingredients to the dry and stir until the batter just comes together. Add the blueberries (use other fruit if you like! berries, chopped apples, diced peaches, pears, etc. are all wonderful), and fold them in gently.
Pour the batter onto the sheet pan and smooth it into an even layer with your spatula. It's a scant amount of batter, so you don't have to take it all the way to the edges, just enough to keep it in a thin but not too thin layer.
Sprinkle the top of the batter liberally with turbinado sugar and bake for about 20 minutes, or until just turning golden brown on the edges. Remove from the oven, let cool for at least 10 minutes, then slice into squares.