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Preheat oven to 350 degrees. Grease (or line with paper baking cups) 12 standard muffin cups.
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To make the streusel, combine sugar, brown sugar, flour, and cinnamon- stir to combine. Add cubed butter and with a pastry cutter or two forks, cut butter into sugar mixture until you have pea-size crumbles. Set aside.
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In a large bowl, whisk together flour, sugar, baking powder, baking soda, and salt. Set aside.
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In another bowl, whisk egg and then add buttermilk, melted butter, and vanilla. Whisk to combine.
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Add buttermilk mixture to flour mixture and gently fold until just combined. Fold in blueberries.
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Spoon batter evenly into prepared muffin cups, and sprinkle streusel evenly to the top of batter.
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Bake 18-2o minutes, or until a toothpick inserted in the center of muffin comes out clean.
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Remove from oven and immediately remove muffins from pan. Serve warm or let cool on a wire rack and store in airtight container for up to 4 days.
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To reheat, wrap a wet paper towel around muffin and microwave on high for 10 seconds.