Whisk grits into boiling water in a metal bowl, then set bowl over a saucepan of simmering water. Cook, stirring constantly, 2 minutes.
Stir in 1 cup milk and 3/4 teaspoon salt and cook, stirring constantly, 15 minutes. Grits will be very thick.
Remove bowl from heat and stir in remaining cup milk and butter. Return bowl to simmering water and cook, stirring, until smooth and heated through, about 2 minutes.
Remove bowl from heat and whisk in yolks, then cool to lukewarm.
Beat egg whites with remaining 1/4 teaspoon salt using an electric mixer until they just hold stiff peaks, then fold into grits gently but thoroughly.
Turn mixture into a buttered 2 1/2- to 3-quart baking dish and bake in middle of oven until browned and puffed, 40 to 45 minutes.
• You can use a double boiler to cook grits, but we think it's easier to stir them in a metal bowl as they thicken.
