1. Adjust oven rack to the middle position. Preheat oven to 375°F. Lightly grease and flour forty-five 1¾-inch muffin cups*; set aside.
2. In a medium bowl, whisk egg for 30 seconds.
3. Add 1 cup sugar, the vanilla and 1 teaspoon lemon zest. Whisk mixture for 30 seconds or until thick. Slowly whisk in ¼ cup melted butter. Add buttermilk and whisk until combined.
4. In a large bowl, stir together 2 cups flour and 1 tablespoon baking powder. Fold in egg mixture until nearly combined. Toss berries with 2 tablespoons flour; add to egg mixture, folding to combine.
5. Spoon batter into prepared muffin cups, filling three-fourths full. For streusel, stir together remaining ⅔ cup sugar, 2 tablespooons flour, 2 teaspoons lemon zest and ¼ teaspoon baking powder. Sprinkle mixture on top of muffins.
6. Bake for 10 to 12 minutes or until golden. Cool in muffin cups on a wire rack for 5 minutes. Remove from muffin cups and serve warm.

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*Note: If your mini muffin pan doesn’t hold this quantity, bake the muffins in batches.

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