1/3 c sugar
1 1/2 t grated lemon zest
2 c fresh or frozen blueberries
1 1/8 cups + 1 t sugar
2 1/2 c flour
2 1/2 t baking powder
1 t salt
4 T (1/2 stick) unsalted butter, melted and cooled slightly
1/4 c vegetable oil
1 c buttermilk (or 3/4 c plain yogurt + 1/4 c milk)
1 1/2 t vanilla
For the topping: Stir together the sugar and lemon zest in a small bowl and set aside.
For the muffins: Adjust an oven rack to the upper-middle position and heat the oven to 425*. Spray a muffin tin with cooking spray. Put 1 c of blueberries and 1 t sugar in a small saucepan and bring to a simmer over medium heat. Cook, mashing the berries with a spoon several times and stirring frequently, until the berries have broken down and the mixture is thickened and reduced to 1/4 c, about 6 minutes. Transfer to a small bowl and let cool to room temperature, 10-15 minutes.
Whisk flour, baking powder and salt together in a large bowl. Whisk in remaining 1 1/8 c sugar and eggs together in a medium bowl, until thick, about 45 seconds. Slowly whisk in melted butter and oil until combined. Whisk in buttermilk and vanilla until combined. Using a rubber spatula, fold in the egg mixture and the remaining 1 c blueberries into the flour mixture until just moistened (batter will be quite lumpy).**
Use and ice cream scoop to divide the batter equally into the prepared muffin cups. The batter should completely fill the cups. Spoon 1 t of the cooked berry mixture onto the center of each muffin cup. Using a skewer, gently swirl the berry filling into the batter, using a figure * motion. Sprinkle the lemon sugar over the muffins.
bake until muffin tops are golden and just firm, 17-19 minutes, rotating the Muffin tin halfway through the cooking time. Cool for 5 minutes, then remove from the tin and finish cooling on a wire rack.
** if using frozen berries, toss them in the flour mixture first, before adding the egg mixture.