For the streusel topping:
1/4 cup packed brown sugar
2 3/4 tablespoons all purpose flour
2 tablespoons cold butter, cut into small pieces
For the batter:
2 1/2 cups all purpose flour
2 teaspoons baking soda
1/4 teaspoon salt
1 egg, beaten
3/4 cup granulated sugar
2/3 cup olive oil
1 teaspoon pure vanilla extract
1 teaspoon lemon zest (grated peel)
1 cup milk
1 tablespoon white vinegar
1 3/4 cups fresh or frozen blackberries (I used Marionberries, but any type of blackberry will do)
1. Preheat the oven to 350 degrees F. Grease the cups of a muffin tin (or line with paper); set aside. If using fresh blackberries, cut them in half.
2. Make the streusel: In a small bowl, mix together the brown sugar and 2 3/4 tablespoons flour. Add the butter and mix well with your hands until the mixture resembles coarse bread crumbs. Set aside.
3. Make the batter: In a large mixing bowl, combine the 2 1/2 cups flour, baking soda, and salt. Make a well in the center of the mixture, and add the egg, granulated sugar, oil, vanilla extract, and lemon zest. Stir into the flour mixture. Add the milk and vinegar and stir until just moistened. Fold in the berries.
4. Spoon the batter into the muffin cups, coming nearly to the top of each cup. Sprinkle with the brown sugar streusel. Bake for 20 - 30 minutes, or until a toothpick inserted in the center of a muffin comes out clean. Cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.