Add the egg and vanilla extract, mixing until combined.
Add the flour, baking soda, salt, and lemon zest. Mix until a dough forms, stop mixing as soon as the ingredients are incorporated.
You’ll notice the dough is soft, pliable, and a little sticky. That’s perfect, it makes it easier to knead in the blueberries. Gently fold in the blueberries by hand.
If the dough feels too sticky, chill it in the fridge for a short while. Otherwise, go ahead and roll the dough into balls and place them on a parchment-lined baking sheet. Flatten them slightly.
Bake at 375°F/190°C (conventional oven) for about 15 minutes, or until the edges are golden brown.
How to store
Let the cookies cool on the baking sheet first, then transfer them to a wire rack to cool completely. Store in an airtight container for up to a week.