1. Preheat the oven to 425°F (220°C). Place the unsalted butter in an 8×8-inch baking dish or similar oven-safe pan and put it in the oven while it preheats, just until the butter is fully melted and lightly bubbling. Carefully remove the pan and set it aside.

2. In a large mixing bowl, whisk together the all-purpose flour, baking powder, granulated sugar, and fine salt until well combined and evenly distributed.

3. Pour the cold buttermilk into the dry ingredients, then add the vanilla extract and lemon zest. Stir gently with a spatula or wooden spoon just until a thick, slightly lumpy batter forms and no dry pockets of flour remain.

4. Gently fold the blueberries into the batter, being careful not to crush them too much so they stay mostly whole and distribute evenly throughout.

5. Pour or spoon the batter over the hot melted butter in the baking dish, spreading it into an even layer. Do not stir the butter into the batter; let it pool around the edges and on top.

6. Use a knife or bench scraper to lightly score the batter into 9 even squares. Sprinkle the coarse or sanding sugar generously over the surface of the batter.

7. Bake for 22–26 minutes, or until the biscuits are puffed, the top is deeply golden, and the butter is bubbling around the edges. A toothpick inserted in the center should come out mostly clean with just a few moist crumbs.

8. Let the biscuits cool in the pan for about 10 minutes to allow the blueberry juices to thicken slightly. Cut along the scored lines, lift out the squares, and spoon some of the buttery blueberry sauce from the bottom of the pan over each serving. Serve warm.