In a small bowl, whisk together the flour, sugar, and cinnamon. Pour in the melted butter and stir with a fork until small crumbles form. Some larger clumps are fine. Set aside.

Position an oven rack in the upper third of the oven (just above center) and preheat to 400°F. Line a 12-cup standard muffin tin with tulip-style liners. If using standard liners, you will get more than 12 muffins. If not using liners, generously spray each cup with a flour-based baking spray.

In a medium bowl, whisk together the flour, baking powder, baking soda, and salt. Set aside.

In a separate large bowl, whisk the eggs and sugar until pale and slightly thickened. Whisk in the melted butter, lemon zest, and lemon juice. Then whisk in the buttermilk and vanilla until smooth.

Add the dry ingredients to the wet mixture in three additions, folding gently with a spatula until just combined—a few streaks of flour are okay. Gently fold in the blueberries. For extra insurance against sinking, toss the blueberries in 1 tablespoon of flour before adding.

Using a 3-tablespoon cookie scoop or two spoons, divide the batter evenly among the prepared muffin cups, filling tulip liners all the way to the top. (For standard liners, fill only two-thirds full.) Sprinkle the streusel topping generously over each muffin.

Bake for 18 to 20 minutes, until golden brown and a toothpick inserted into the center of a muffin comes out clean. Let the muffins cool in the pan for 5 minutes, then transfer to a wire rack.

Serve warm with a pat of butter, if desired. These muffins are also wonderful at room temperature.