These came from Cooks’ Illustrated. Cooking some of the berries in sugar and adding our cooled homemade jam to the batter along with fresh, uncooked berries gave our muffin the best of both worlds; intense blueberry flavor and the liquid burst that only fresh berries can provide.
Stir together sugar, brown sugar, and flour in a small bowl until combined. Drizzle with warmbutter and toss with a fork until evenly moistened and mixture forms large chunks with somepea-sized pieces throughout. Set aside.
Muffins
Adjust oven rack to upper-middle position and heat oven to 425 degrees. Spray standard muffin tin with nonstick cooking spray. Bring 1 cup blueberries and 1 teaspoon sugar to simmer in small saucepan over medium heat. Cook, mashing berries with spoon several times and stirring frequently, until berries have broken down and mixture is thickened and reduced to 1/4 cup,about 8 minutes. Transfer to small bowl and cool to room temperature, 10 to 15 minutes.
Whisk flour, baking powder, and salt together in large bowl. Whisk remaining 1 1/8 cup sugar and eggs together in medium bowl until thick and homogeneous, about 45 seconds. Slowly whisk in butter and oil until combined. Whisk in buttermilk and vanilla until combined. Using rubber spatula, fold egg mixture and remaining cup blueberries into flour mixture until just moistened. (Batter will be very lumpy with few spots of dry flour; do not overmix)
Use ice cream scoop or large spoon to divide batter equally among prepared muffin cups (batter should completely fill cups and mound slightly). Spoon teaspoon of cooked berry mixture into the center of each mound of batter. Using chopstick or skewer, gently swirl berry filling into batterusing figure-eight motion. Sprinkle streusel topping evenly over muffins.
Bake until muffin tops are golden and just firm, 17 or 19 minutes, rotating muffin tin from front to back halfway through baking time. Cool muffins in muffin tin for 5 minutes, then transfer to wire rack and cool 5 minutes before serving.
Notes
If buttermilk is unavailable, substitute 3/4 cup plain whole-milk or low-fat yogurt thinned with 1/4 cupmilk.