Preheat oven to 425. Spray muffin tin thoroughly. In small saucepan heat 1 cup of blueberries with 1 tsp sugar. Bring to a simmer and cook, mashing until popped and thickened, about 5 minutes. Transfer to small bowl and set aside to cool.

In large bowl whisk flour, baking powder and salt. In separate bowl whisk 1 1/4 cup sugar and eggs till thick. Slowly whisk in oil and butter till combined. Add buttermilk and vanilla and whisk until combined.

Fold the egg mixture into the dry ingredients along with the remaining blueberries. Stir just until moistened. Divide into muffin cups. Spoon half teaspoon of blueberry mixture into center of each cup. Using a skewer or toothpick gently swirl the filling into the batter.

Sprinkle the lemon sugar mixture over the top of each muffin. Bake for 17-19 minutes. Cool in pan for 5 minutes, then transfer to cooling rack.