Put oven rack in middle position and preheat the oven to 400°F.
Add milk to a heavy saucepan. To ensure the batter has a smooth consistency, combine ¼ cup granulated sugar and corn starch together in a small bowl. Then add it to the milk, along with the grapefruit zest. Bring to a boil over medium heat, whisking constantly for about 1 minute. Remove from heat and whisk in the unsalted butter, grapefruit juice, and liqueur until butter is melted, then whisk in the egg yolks until blended to make a pastry cream.
In a large bowl, beat the egg whites with the salt and cream of tartar using an electric mixer at medium speed, or a whisk until they hold soft peaks. Add 3 tbsp. of granulated sugar a little at a time, beating until whites hold stiff peaks.
Rub a thin layer of cold butter along the entire interior of a 9” pie pan or 6 individual 1 cup ceramic soufflé ramekins. Coat in sugar, shaking off excess sugar. You may do this ahead of time and keep refrigerated.
Gently and thoroughly fold egg whites into the grapefruit mixture to lighten. Spread in pie plate with a rubber spatula and bake until puffed and golden, 16 to 18 minutes. If using ramekins, spoon mixture 2/3 full into ramekins and bake 10 to 12 minutes. Dust top with powdered sugar and serve immediately.