Add butter to a large saucepan over medium heat. Cook for 4-5 minutes, stirring frequently. The butter will start to bubble, then foam, then subside. When the bubbles subside, the butter should look brown. Pour it into a bowl and let cool completely in the fridge.
Preheat oven to 350. When the butter is chilled, add it to the bowl of a stand mixer with brown sugar and white sugar. Mix for 2 minutes until combined and fluffy.
Add the eggs and vanilla and mix again. Add flour, cornstarch, baking soda, and salt. Mix until just combined. Add lemon zest and juice, and blueberries. Gently mix until blueberries are evenly mixed in the dough.
Using a large cookie scoop, scoop the dough onto a lined baking sheet. Bake at 350 for 9-12 minutes, until cookies are just set. Let them cool to set up and enjoy!