preheat the oven to 350°

line a baking sheet with parchment or foil as needed, set aside

mix streusel by using a fork to combine all the ingredients until crumbly, set aside

beat the butter and sugar together for 3-5 minutes until fluffy

beat in the egg, vanilla, and lemon zest if using

in a separate bowl mix together the flour, soda, and salt

stir into the butter mixture just to combine

stir in the frozen or fresh blueberries

divide the cookie dough into 10 pieces, roll lightly into balls

lay on the prepared cookie sheet, lightly flatten the dough balls

divide the streusel between the cookies and pat lightly to press it into the cookie dough

bake in the preheated oven for 11-12 minutes or until lightly browned

cool on the baking sheet 4-5 minutes before removing to a baking rack