2 cups fresh blueberries, picked over, washed and dried
1 1/8 cups sugar plus 1 teaspoon
2 1/2 cups all purpose flour
2 1/2 teaspoons baking powder
1 teaspoon salt
2 large eggs
4 tablespoons unsalted butter (1/2 cup), melted and slightly cool
1/4 cup vegetable oil
1 cup buttermilk
1 1/2 teaspoons vanilla
Lemon Sugar Topping:
1/3 cup sugar
1 1/2 teaspoons fresh zest of a lemon
Mix together the sugar and lemon zest and set aside until ready to bake muffins.
For the muffins: Adjust the oven rack to the upper middle position and preheat oven to 425 degrees.
Line a 12 muffin tin with muffin papers or spray with non-stick baking spray.
Bring 1 cup of the blueberries and 1 teaspoon of the sugar to simmer in a small saucepan over medium heat. Cook, mashing berries with spoon several times, stirring frequently, until berries have broken down and mixture is thickened and reduced to 1/4 cup, about 6 minutes. Transfer to a small bowl and cool to room temperature (takes about 15 minutes).
Whisk the flour, baking powder, and salt together in a large mixing bowl.
Whisk remaining 1 1/8 cups sugar and eggs together in a medium bowl until thick well blended, about 45 seconds. Slowly whisk in butter and oil until combined. Whisk in buttermilk and vanilla until combined. Use a rubber spatula and fold the egg mixture and remaining cup of blueberries into the flour mixture JUST until moistened. (Batter will be lumpy - don't overmix).
Spoon batter into paper muffin cups, completely filling cups. Place 1 teaspoon of the cooked berry mixture into the center of each batter-filled muffin cup, and swirl into the batter with a skewer, spreading the berry flavor throughout. Sprinkle lemon zest/sugar mixture over each muffin. Bake until muffin tops are golden brown and just firm, 17 to 19 minutes, rotating muffin tin front to back halfway through baking time. Cool muffins in the tin for 5 minutes, then transfer to a wire rack and cool 5 additional minutes before serving.