1. Wash and rinse blackberries under water. Set aside.
2. Preheat oven to 425°. Grease your muffin pan with Crisco or cooking spray.
3. In a medium sized bowl, sift together flour. Measure ¼ C of the sifted flour and pour over blackberries. Gently stir to coat the blackberries in flour.
4. Take your remaining flour and add sugar, baking powder and salt. Set aside.
5. In a medium size bowl, beat the egg using a whisk until light and fluffy. Add melted butter and whisk together.
6. Continue to add vanilla extract, buttermilk and sour cream and whisk until smooth. Add fresh squeezed lemon juice and zest.
7. Slowly pour wet ingredients into the dry ingredients and use a wooden spoon to mix just until combined. Your batter will be thick, but be careful not to over mix! Fold in the blackberries last.

For the Streusel:

1. Combine melted butter with all dry ingredients. Use a pastry cutter or fork to cut the butter into the dry ingredients until no large chunks remain.
2. Fill your muffin tins all the way to the top with batter. Add 1 tablespoon of streusel to the top of each muffin.
3. Bake for 5 minutes at 425°, then reduce heat to 350° and bake for another 16-20 minutes. Do not open the oven for at least 15 minutes. Baking times will vary by oven.
4. Check your muffins for doneness by inserting a toothpick into the center of the muffin. If your toothpick comes out clean, your muffin is done. Allow muffins to cool completely in the pan.

For the Glaze:

1. Combine powdered sugar with lemon juice and milk. Whisk until smooth. Add additional powdered sugar to thicken or additional milk to make thinner.
2. Drizzle glaze over cooled muffins.