Prep the pan and oven: Heat the oven to 350°F (175°C).
Grease a 9x13-inch baking dish with butter or oil. If you like a golden top, you can line the bottom with parchment for easy lifting.
Combine dry ingredients: In a large bowl, whisk the flour, baking powder, baking soda, salt, sugar, and cinnamon (if using). Whisk well so the leaveners are evenly dispersed.
Mix the wet ingredients: In a separate bowl or large measuring cup, whisk the buttermilk, eggs, vanilla, and melted butter.
Make sure the butter is slightly cooled so it doesn’t scramble the eggs.
Bring it together: Pour the wet mixture into the dry ingredients. Stir gently with a spatula until just combined. A few small lumps are fine.
Avoid overmixing; it can make the bake tough.
Add blueberries and zest: Fold in the blueberries and lemon zest. If using frozen berries, toss them in 1 teaspoon of flour first to reduce bleeding. Work quickly so the batter stays airy.
Pour and top: Spread the batter into the greased pan, smoothing the top.
Sprinkle the optional tablespoon of sugar over the surface for a light crust.
Bake: Place on the center rack and bake for 28–35 minutes, or until the top is golden and a toothpick inserted in the center comes out clean or with a few moist crumbs.
Rest and serve: Let the bake rest for 10 minutes. Slice into squares and serve warm with maple syrup, a pat of butter, or a dusting of powdered sugar.