Steps

• Set the oven rack to the upper middle position. Preheat to 425ºF (218ºC).
• Melt the butter in a pan or microwave, then cool to room temperature.
• In a large bowl, whisk together the flour, baking powder, and salt.
• In a medium bowl, whisk together sugar and eggs until smooth and pale yellow in color, about 1 minute. Slowly whisk in the cooled melted butter and vegetable oil until incorporated. Whisk in the buttermilk and vanilla.
• Add the egg mixture and 1 ¼ cups of blueberries to the flour mixture. Gently fold the mixture until the flour is just moistened, with some lumps and flour spots remaining. Do not overmix the muffin batter.
• Grease the muffin pan with nonstick cooking spray or vegetable oil or line it with paper cups. Fill each opening to the top, about a ⅓ cup per muffin. Sprinkle some sugar over the top of each muffin. Evenly distribute the remaining ¼ cup of blueberries on top, lightly pressing in 2 to 3 berries per muffin.
• Bake for 10 minutes. Rotate the pan and bake until golden brown, 6 to 7 minutes.
• Cool the muffins in the pan for 5 minutes. Use a spatula to carefully remove them, then transfer them to a wire rack until ready to serve.