This lush summer dessert comes together quickly and impresses friends and family.
In a small bowl, sift together flour, cornstarch, and sugar.
In another bowl, beat egg and yolk until pale and fluffy. Add dry ingredients slowly; beat until pale and fluffy again, about 2 minutes.
In a small, heavy-bottomed stainless-steel saucepan, scald milk. Slowly add half of it to egg-flour mixture, beating constantly. Add this mixture to remaining milk in saucepan, and cook over a very low flame, whisking constantly. When mixture bubbles and thickens, remove from heat, and continue whisking for 1 minute. Transfer to a bowl, and stir in vanilla and butter. Refrigerate until chilled. (The dessert can be prepared to this point up to 2 days ahead.)
Preheat broiler. Fold mascarpone into custard and divide among gratin dishes. Press berries into custard. Sift confectioners' sugar over tops and set on a cookie sheet. Broil on top rack until evenly browned, about 2 minutes. Let cool 1 minute and serve.
Use four 4-inch gratin dishes for this custard, and make sure the broiler is fully heated before browning the tops.