Preheat the oven to 425°F. Line every other muffin tin in a standard size muffin tin with muffin liners (it should look like a checkerboard), then spray the top of the tin with cooking spray. Set aside.
In a large mixing bowl, add the melted butter and granulated sugar. Use a silicone spatula to mix until combined.
Add the eggs, Greek yogurt, buttermilk and vanilla extract and mix well.
Add the flour, baking powder, and salt. Mix until combined and no streaks of flour remain. Use the silicone spatula to gently fold in the blueberries.
Scoop the batter evenly into the muffin cups (you may have to bake in batches). If you want to get fancy, use a 3 ounce cookie scoop to create perfectly-domed scoops. Top with demerara sugar, if using.
Bake the muffins for 5 minutes at 425°F, then reduce the heat to 375°F. Continue baking until the tops of the muffins are golden brown and spring back to the touch, about 18 additional minutes.
Place the pan on a cooling rack and allow the muffins to cool slightly in the pan before removing and serving.