Desserts that can be made ahead of time are ideal, and this is one of those. It looks incredibly elegant-golden puffed squares of pastry, filled with a tangy whipped cream, and heaped with fresh summer strawberries. But all the components can be made ahead in very little time and assembled in a snap. Your guests will never know how easy these are-only that they want another one.
Heat the oven to 400 degrees F. Roll the sheet of puff pastry out on a floured countertop and cut into 8 squares. Repeat with the second sheet. Place the squares on two baking sheets lined with parchment or Silpats. Bake for 15 minutes, or until the squares are puffed and golden. Remove the baking sheet from the oven and let the pastries cool for 5 minutes. While the pastries are still warm, press the centers down with the back of a spoon, creating an indented well. Let the pastry cool. (They can be made 24 hours in advance and stored in an airtight container at room temperature after cooling.)
Using a stand mixer, hand mixer, or stick blender, whip the whipping cream and sour cream for about 5 minutes or until it has thickened considerably and holds soft peaks. Whip in the powdered sugar, vanilla and salt. Chill until ready to assemble the tarts. (This filling and the berries can also be prepared up to 24 hours ahead of time.)
Toss the sliced strawberries with the balsamic vinegar and the white sugar. Refrigerate until ready to assemble the tarts.
When ready to serve the tarts, place a spoonful of the whipped sour cream in the center of a puff pastry tartlet shell. Spoon a few strawberry slices on top, and sprinkle with turbinado sugar. Garnish with a mint sprig. Repeat with the other tartlets and serve immediately. Makes 16.