Lemon-Sugar Topping: (I doubled this topping so I wouldn't have to skimp on it when distributing among the muffins)
1/3 cup sugar
zest from one lemon or orange
For the topping, stir together sugar and lemon zest in a small bowl and set aside.
2 cups frozen blueberries
1 1/8 cups plus 1 tablespoon sugar
2 1/2 cups all-purpose flour
2 1/2 teaspoons baking powder
1 teaspoon salt
2 large eggs
4 tablespoons butter, melted and cooled slightly
1/4 cup oil
1 cup buttermilk
1 1/2 teaspoons vanilla extract
Preheat oven to 375 degrees. Spray a standard muffin tin with nonstick cooking spray. Rinse one cup frozen berries under cold water and spread on a double layer of paper towels to dry well. Bring remaining one cup berries and 1 teaspoon sugar to simmer in a small saucepan over medium heat. Cook, mashing berries with a spoon, stirring frequently until the berries have broken down and the mixture is thickened and reduced to about 1/3 cup, about 6-8 minutes. Transfer to a small bowl and cool to room temperature, about 10 to 15 minutes.
Whisk the flour, baking powder, and salt together in a large bowl. Toss the berries that have dried on the paper towels in with the flour mixture. Whisk remaining 1 1/8 cups sugar and eggs together in a medium bowl until thick. Slowly whisk in the butter and oil until combined. Whisk in the buttermilk and vanilla until combined. Using a rubber spatula, fold egg mixture into flour mixture until just moistened. The batter will be lumpy with a few dry spots of flour – don’t overmix.
Fill the muffin cups until they are about 3/4 full. Spoon a teaspoon of cooked berry mixture into the center of each mound of batter. Using a skewer (or a chopstick), gently swirl the berry filling into the batter using a figure eight motion. Sprinkle lemon sugar evenly over the muffins.
Bake until the top of the muffins are golden and just firm, 17-18 minutes. Cool muffins in the muffin tin for 5 minutes, then transfer to a wire rack and cool 5 minutes before serving.
Recipe adapted from Mel's Kitchen Cafe recipe blog: