2 c. fresh blueberries
1 1/8 c. unbleached all-purpose flour
2 1/2 t. baking powder
1 T. salt
2 large eggs
4 T. (1/2 stick) unsalted butter, melted and cooled slightly
1/4 c. vegetable oil
1 c. buttermilk
1 1/2 t. vanilla extract
For the topping: Stir together sugar and lemon zest until combined; set aside.
For the muffins: Preheat oven to 425 degrees. Bring 1 cup blueberries and 1 teaspoon sugar to simmer over medium heat. Cook, mashing berries with a spoon until berries have broken down and mixture is thickened and reduced to 1/4 c. Cool to room tempature in a small bowl.
Whisk flour, salt, and baking powder together in a large bowl. Whisk 1 1/8 c. sugar and eggs togther until mixture is homogeneous. Slowly whisk in butter and oil until combined. Whisk in buttermilk and vanilla until combined. Using rubber spatula, fold egg mixture and remaining blueberries into flour mixture until just moistened. (Batter will be lumpy, but do not overmix.)
Divide batter equally among 12 muffin cups (fill cups completely). Spoon teaspoon of cooked berry mixture into center of each mound of batter. Using a chopstick or skewer, gently swirl berry filling into batter using figure-eight motions. Sprinkle lemon sugar evenly over muffins.
Bake 17 to 19 minutes.