Preheat oven to 400 degrees Fahrenheit. Line a baking sheet with parchment paper.

Place four pastry squares on baking sheet. Poke holes with the tines of a fork all over each square.

Bake pastry for five minutes. Remove from oven and cool slightly.

Cover the surface of each square with a layer of grapes, leaving a small border of dough all around. (If grapes are large, cut them in half.)

Cut one of the unbaked pastry squares into four even-sized strips. Place cut strips along the outer edges of a grape-covered square, forming a raised edge. Overlap dough strips at the corners and cut off any excess dough. Press down gently on dough strips along edges of each tart to secure them.

Repeat with remaining squares. Brush the top of dough lightly with egg yolk.

Bake 15 minutes longer.

Remove tarts from oven. Brush grapes with a small amount of dessert wine and sprinkle with sugar. Serve at room temperature with whipped cream.