Add the butter, flour, and sugar to a bowl. Use a pastry blender to incorporate until the mixture is crumbly. Add the egg yolks, one at a time, mixing well after each addition. Mix with your hands (or a wooden spoon) until incorporated. Add the milk and continue to mix until the dough forms a ball. (Add more or less milk as needed.)

Form the dough into a disc shape and wrap it in plastic wrap. Refrigerate for one hour.

In the meantime, prepare the filling by adding the blueberries, demerara sugar, flour, lemon zest, and lemon juice. Toss to coat.

Preheat your oven to 350 degrees F.

Roll out the dough to about ⅛” thickness. Transfer the dough to a parchment-lined baking sheet. Add the filling to the center leaving a two-inch border around the outside. Gently fold the border of dough over the filling. Brush the dough with egg wash. Sprinkle the dough with the remaining demerara sugar.

Bake for 40-45 minutes or until the crust is golden and the filling is bubbling. Allow to cool. Serve warm or at room temperature and top with vanilla ice cream (optional, but recommended).