From: Art of Gluten-Free Baking (http://artofglutenfreebaking.com)
-Best Blueberry Muffins, Gluten-Free
-adapted from May/June 2009 Cook’s Illustrated
Lemon Sugar Topping (optional)
Prepare your muffin pans--I use muffin papers in mine. You can also grease cups with extra butter and flour with extra tapioca flour.
For the jam: in a small saucepan, bring 1 cup blueberries and 1 tsp sugar to simmer over medium heat. Cook, mashing berries and stirring frequently, until berries have broken down and mixture is thickened and reduced to about 1/4-1/3 cup, about 6 minutes (don't get too worried about the exact measurement). Transfer to small bowl and cool to room temperature--about 10-15 minutes.
In a small bowl, mix together flour, baking powder, and salt.
In a large bowl, beat eggs for a couple a minutes--until light. Add remaining 1 1/8 cup sugar and beat for about 3 minutes more. Slowly add melted and cooled butter and oil, beat for a few minutes more. Add vanilla, beat to combine.
Now add flour mixture and buttermilk, alternating between the two--beginning and ending with the flour mixture. With a large spoon, fold in the remaining 1 C of whole blueberries.
Divide the batter equally among 12 prepared muffin cups. The batter should fill each cup and mound slightly. With a teaspoon, spoon the jam onto the top of each batter-filled mound. Using a chopstick (or something similar), gently swirl the jam into the batter.
If using, sprinkle a bit of lemon sugar topping onto each mound of batter. You can also just sprinkle plain granulated sugar on each. Or not use any topping. All options are delicious!
Bake at 425 degrees F for about 19 minutes--until tops are golden brown. Cool in pans for 5 minutes and then remove to wire racks to cool completely.