Recipe Source: Modified slightly from my friend Anisha who got it originally from Cook's Illustrated
**Cook's note-These muffins can be flavored with any variety of fruits and toppings. Using the muffin recipe as a base for any flavor you like!**
Edited 12/24/11--Add your butter gradually if you decide to do the streusel topping. More or less may be needed depending on the humidity in your area to create a nice crumbly topping. If you find it is too soft add more flour to achieve the consistency you need.
1/3 cup (21/3 ounces) sugar
1½ teaspoons finely grated zest from 1 lemon
3 T white sugar
3 T brown sugar
1/3 cup flour
5 T melted butter
2 cups (or less if you are me!) fresh blueberries, picked over
1 1/8 cups plus 1 teaspoon sugar
1 tablespoon water
2 1/2 cups unbleached all-purpose flour
2 1/2 teaspoons baking powder
1 teaspoon table salt
2 large eggs
4 tablespoons (½ stick) unsalted butter, melted and cooled slightly
¼ cup vegetable oil
1 cup buttermilk
2 teaspoons vanilla extract
If topping with lemon sugar, stir together sugar and lemon zest in small bowl until combined; set aside.
If topping the muffins with streusel, combine ingredients until it is the size of peas and set aside.
Adjust oven rack to middle position and heat oven to 425 degrees. If your oven cooks on the hot side normally, reduce the heat. Some people have commented that 425 was too hot. I would recommend getting a thermometer for your oven if you aren't sure.
Prepare standard muffin tins with nonstick cooking spray or liners. Bring 1 cup blueberries, water, and 1 teaspoon sugar to simmer in small saucepan over medium heat. Cook, mashing the berries with your potato masher or fork several times and stirring frequently, until berries have broken down and mixture is thickened and reduced by about half. This will take about 6 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 sugar and eggs together in medium bowl until thick and well combined. Slowly mix in butter and oil until combined. Whisk in buttermilk and vanilla. 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 over mix.)
Using a cookie scoop or large spoon, divide batter equally among prepared muffin cups (batter should completely fill cups and mound slightly). Spoon teaspoon of cooked berry mixture into center of each mound of batter. Using chopstick or skewer, gently swirl berry filling into batter using figure-eight motion. Sprinkle lemon sugar or streusel evenly over muffins.
Bake until muffin tops are golden and just firm, 17 to 19 minutes. Cool muffins in muffin tin for 5 minutes, then transfer to wire rack and cool 5 minutes before serving.