Start by letting your butter and eggs sit out for about an hour to reach room temperature. Line two baking sheets with parchment paper and preheat your oven to 350°F.
For the crumble topping, combine the flour, brown sugar, and cinnamon in a bowl. Add the cold butter cubes and work them in with your fingertips. Pop this in the fridge while you make the cookie dough.
In a medium bowl, whisk together the flour, baking powder, and salt.
In your large mixing bowl, beat the softened butter until it’s creamy – about 30 seconds. Add the sugar and beat until it’s light and fluffy, about 2-3 minutes.
Beat in the eggs one at a time, letting each one fully mix in before adding the next.
Add the lemon zest, lemon juice, and vanilla. The mixture might look a bit curdled at this point, It’ll smooth out when we add the flour.
Gradually stir in the flour mixture until just combined. The dough will be soft but not sticky. Gently fold in the blueberries.
Using a 2-tablespoon cookie scoop or spoon, drop rounded portions onto your prepared baking sheets, leaving about 2 inches between each cookie. They’ll spread a bit while baking.
Take your crumble mixture from the fridge and sprinkle a generous amount on top of each cookie, gently pressing it in just a bit so it sticks.
Bake for 13-15 minutes, rotating the pan halfway through. Look for light golden edges and a set center – the cookies should be just barely starting to brown around the bottom edges.
Let them cool on the baking sheet for 5 minutes – they’re quite delicate when hot. Then transfer to a wire rack to cool completely.