• Preheat the oven to 400 °F (200 °C) and line a standard muffin pan with liners. This recipe yields about 15 muffins.

Make the streusel:

• In a small bowl, mix the flour, brown sugar and salt until combined.
  2/3 cup (83 grams) flour, 2/3 cup (133) light brown sugar, 1/4 teaspoon salt
• Use cold butter cut into small cubes and rub it into the dry ingredients with your fingertips until it looks like wet sand. You’re aiming for coarse, sandy crumbs rather than large doughy clumps. Set aside.
  1/4 cup (56) cold unsalted butter

Make the muffins:

• In a medium-sized bowl, sift or whisk together the flour, baking powder, and salt until well combined.
  2 cups (250 grams) flour, 2 teaspoons baking powder, 1 teaspoon salt
• In a separate large bowl, whisk the sugar, eggs, milk, oil, lemon zest, vanilla and lemon extracts until the mixture is smooth.
  1 cup (200 grams) granulated sugar, 2 large eggs, 3/4 cup (177 ml) milk of choice, 1/2 cup (120) canola or vegetable oil, 1 tablespoon lemon zest, 1 1/2 teaspoons vanilla extract, 1 1/2 teaspoons lemon extract
• Pour the dry flour mixture into the wet ingredients. Gently fold them together and get rid of any huge lumps. Smaller lumps are okay.
• Cut the blackberries in half, leaving any smaller blackberries whole. If you’re using frozen berries, you don’t need to let them thaw to cut them.
  2 cups (272) fresh or frozen unthawed blackberries
• Toss the berries with the extra 2 teaspoons of flour, then gently fold it into the batter, leaving any excess flour behind in the bowl.
• Divide the batter evenly among the cups, filling them about 4/5 full (about 77 grams each). Sprinkle a generous amount of streusel (18 grams - a heaping tablespoon) over each muffin.
• Bake for 18-22 minutes (closer to 18 for fresh blackberries and 22 for frozen) or until a toothpick inserted into the center comes back with just a few crumbs, but no liquid batter.
• Let the muffins rest in the hot pan for 10 minutes and then carefully transfer them to a wire rack to cool completely, about 1 hour.

Make the glaze:

• In a small bowl, whisk together the powdered sugar, 1 1/2 tablespoons of lemon juice and lemon zest until velvety smooth. If you prefer a thinner drizzle, whisk in more juice a few drops at a time; for a thicker icing, simply stir in an extra spoonful of powdered sugar. Drizzle over the muffins.
  1 cup (120 grams) powdered sugar, 1 1/2-2 tablespoons freshly squeezed lemon juice, 1/2 teaspoon lemon zest
• Store in an airtight container at room temperature for up to 2 days or in the refrigerator for up to 5 days. To freeze, place cooled muffins in a freezer-safe container and freeze for up to 3 months. Thaw at room temperature or warm briefly in the microwave.