Preheat the oven to 375°F. Lightly grease a standard 12-cup muffin tin; or line the tin with papers, and grease the papers.
For Muffins

In a medium sized bowl whisk the eggs with the milk and vanilla extract.
In a large mixing bowl, whisk the flour with the sugar, baking powder, salt, and ground cinnamon. Cut the butter into the flour mixture with a pastry blender or your fingertips. (The mixture should look like coarse crumbs.)
Remove one cup of the mixture and set aside in a separate bowl to make the streusel topping.
Gently fold blueberries and lemon zest in the remaining muffin batter.
Add the milk and egg mixture to the flour mixture. Stir just until combined.
Fill each muffin cup about 3/4 full with the batter, using two spoons or an ice cream scoop.
Streusel Topping

Melt the remaining 2 tablespoons butter and stir into the reserved one cup of flour mixture until it is crumbly and looks like coarse meal. Sprinkle about 1 tablespoon of the streusel on top of each muffin.
Bake the muffins for about 20-25 minutes, until they’re light golden brown on top, and a toothpick inserted into the middle of one of the center muffins comes out clean.
Remove the muffins from the oven, loosen their edges from the pan, and after about 5 minutes transfer them to a rack to cool.
Yield: 12 muffins.
* If using fresh blueberries, dust them lightly with flour; this helps prevent the blueberries from “bleeding” into the muffins. If using frozen blueberries, add them frozen to the batter; this will help prevent them from “bleeding” our their juices.