In a medium bowl, whisk together the all-purpose flour, granulated sugar, and cinnamon.

Add in melted butter and mix until small crumbles form. Big crumbles are cool too. Set aside.

Adjust the oven rack to the 2nd level mark, just above center, and preheat the oven to 400ºF. Line a standard size muffin tin with tulip liners.

In a medium-size mixing bowl, whisk together the all-purpose flour, baking powder, baking soda, and salt. Set aside.

In a separate medium-size mixing bowl, whisk together the eggs, granulated sugar, melted butter, lemon zest, and lemon juice. Then add in the buttermilk and vanilla extract.

Using a spatula, gradually add the dry ingredients to the wet mixture stirring until just combined. Gently fold in the blueberries.

Using a 3 tablespoon-size cookie scoop or two spoons, evenly divide the batter into the muffin liners, filling them to the top.

Top with any extra blueberries (optional) and streusel.

Bake for 18-20 minutes at 400ºF or until an inserted toothpick comes out clean.

Allow to cool for 5 minutes before removing muffins from the tin.

Muffins are best served still warm with a little butter, but are equally just as good completely cooled.