Put oven rack in middle position and preheat oven to 400°F. Butter a 9 1/2-inch deep-dish glass pie plate.
Bring milk, 1/4 cup granulated sugar, cornstarch, and zest to a boil over moderate heat in a 2-quart heavy saucepan, whisking constantly. Boil, whisking, 1 minute.
Remove from heat and whisk in butter, juice, and liqueur until butter is melted, then whisk in yolks until blended.
Beat whites with salt in a large bowl using an electric mixer at medium speed until they just hold soft peaks.
Add remaining 3 tablespoons granulated sugar a little at a time, beating, then beat until whites just hold stiff peaks.
Fold one fourth of whites into yolk mixture to lighten, then fold in remaining whites gently but thoroughly.
Spread in pie plate with a rubber spatula and bake until puffed and golden, 16 to 18 minutes. Dust with confectioners sugar and serve immediately.
