Preheat your oven to 400°F (200°C) and place a rack in the center.

In a small bowl, combine the blueberries, 1 tablespoon sugar, 2 tablespoons flour, and a dash of salt. Toss gently until the berries are evenly coated; set aside.

Let the puff pastry thaw for 40–50 minutes until pliable but still cold.

On a lightly floured surface, cut each pastry sheet into nine equal squares to yield 18 squares total.

Lightly spray a standard muffin tin with cooking spray or brush with a little melted butter.

Press one pastry square into each muffin cup, pushing the corners up the sides. Dock (poke) the center a few times with a fork to prevent the middle from puffing too much.

Spoon the blueberry mixture into each pastry cup, filling about half full.

Place one small cube of butter on top of the berries in each cup. Sprinkle a pinch of sugar over each tart if desired.

Bake for 15–16 minutes, or until the pastry edges are golden brown and the filling is bubbling. Watch the last few minutes to prevent over-browning.

Cool the tarts in the tin for 5–10 minutes, then transfer to a wire rack to cool another 5–10 minutes. Serve warm.