Put oven rack in upper third of oven and preheat oven to 400°F.
Unfold pastry sheet and cut a 3-inch square from pastry with a sharp knife (reserve remaining pastry for another use).
Bake pastry square on an ungreased baking sheet until golden brown, 15 to 20 minutes.
Transfer pastry to a rack and cool completely, then cut pastry horizontally into 2 layers. Stir together crème fraîche and granulated sugar until sugar is dissolved.
Just before serving, put bottom half of pastry on a plate and top with crème fraîche and raspberries, letting some berries spill onto plate. Cover with top half of pastry and dust with confectioners sugar.
Thawed pastry keeps, wrapped first in wax paper and then in plastic wrap, chilled, 2 days.
