Preheat the oven to 425°F (220°C). Grease a standard 12-cup muffin tin with butter or nonstick baking spray.

In a large heatproof bowl, melt ½ cup of butter in the microwave using 15-second intervals on 50% power. Stir until fully melted but not hot. In a separate small bowl, melt the remaining 1½ tablespoons of butter and set aside for brushing later.

Whisk the granulated sugar into the melted butter until combined. Add the eggs and buttermilk, whisking until the mixture is smooth and well incorporated.

Add the flour, baking powder, and baking soda directly into the same bowl. Using the whisk, gently combine the dry ingredients before folding them into the wet mixture with a rubber spatula. Mix only until partially combined—some small lumps are acceptable. Overmixing may result in dense muffins.

Gently fold in approximately 1 cup of the blueberries, reserving a small handful for the muffin tops. Take care not to crush the berries during mixing.

Divide the batter evenly among the prepared muffin cups, filling each about three-quarters full. Place the remaining blueberries on top for even distribution. Bake for 5 minutes at 425°F (220°C), then reduce the oven temperature to 375°F (190°C) and continue baking for 13–16 minutes, or until the tops are golden and a toothpick inserted in the center comes out clean.

While the muffins are still warm, brush the tops with the reserved melted butter and lightly sprinkle with sugar for a subtle crunch. Allow to cool in the pan before transferring to a wire rack.

Store in an airtight container at room temperature for up to 3 days, or refrigerate for up to 4 days. To freeze, place in an airtight container or zip-top bag for up to 3 months. Reheat before serving for best texture.