1. Preheat oven to 325 degrees. Butter seven 4 1/2-ounce pots de creme pots, or six 6-ounce ramekins; set aside. In a large bowl, stir together butter, granulated sugar and salt. Stir in yolks. Add flour, milk, and lemon juice and zest; mix until incorporated.

2. In a separate bowl, beat egg whites until stiff but not dry peaks form. Gently fold egg whites into butter mixture.

3. Ladle batter into prepared pots de creme pots. Transfer pots to a roasting pan. Pour boiling water around pots to come halfway up the sides. Bake until just set and lightly golden, about 20 minutes (25 minutes for ramekins). Remove from roasting pan; let cool slightly. Dust with confectioners' sugar, and serve.