Thaw frozen puff pastry shell according to package directions.
Pre-heat oven to 400 degrees F.
In a small bowl, mix mascarpone cheese with granulated sugar and vanilla. Start with a small amount of sugar. Give it a taste and add more if you like it sweeter. I used about 1 1/2 tablespoons.
Place thawed puff pastry sheet on a lightly floured parchment paper on a baking pan.
Fold in edges of pastry to make a little wall. Be sure all edges are firmly stuck together.
Spread mascarpone mixture in a thin layer in the center of the puff pastry shell. Feel free to take the filling all the way to the inner edge of the pastry walls.
Layer 1 pint of blueberries on top of the mascarpone filling.
Melt 1 tablespoon butter and brush around the edge of the pastry (on the folded wall).
Sprinkle coarse sugar over the top of the blueberries and around the edge of the pastry. This will give the crust an added crunch. If you don’t have coarse sugar, you can skip this step or try a sprinkle of regular sugar.
Bake for 20 minutes or until the crust is puffed and golden brown.
Once cooled, remove pastry from parchment and onto a serving dish or platter. Serve as is or add a dusting of powdered sugar to make it look pretty.