Place the butter in a large saucepan over medium-high heat. Cook until the butter turns golden brown and has a nutty aroma, being careful not to burn, about 10-15 minutes. Lower the heat if you need to. Immediately pour into a bowl and place in the fridge for 45-60 minutes, or leave at room temperature, until it has the consistency of softened butter.

In the meantime, make the filling. Cook the blueberries, lemon zest, lemon juice, sugar, cornstarch and vanilla extract in a shallow pot on medium heat for 15-20 minutes, stirring occasionally until reduced. It helps to smash the blueberries a bit as they break down. Set aside to cool slightly.

Preheat oven to 400ºF. Line a 9×9 pan with parchment, leaving about a 1-inch overhang.

To make the crust, whisk together the flour, sugar and salt. Add in the brown butter and use your fingers to combine, until it forms a crumbly dough that sticks together when pressed.

Press a little over half the dough into prepared pan. Spread the blueberry filling on top. Crumble remaining dough on top of the filling.

Bake for 60-65 minutes, until filling is bubbling and crumble is golden brown. Let cool before slicing into bars. Store wrapped in refrigerator or freezer.