Preheat oven to 350 degrees. Butter six 6-ounce ramekins. Dust with granulated sugar, and refrigerate, uncovered.
Bring juice to a boil in a saucepan. Reduce heat, and simmer until reduced to 1 cup, about 12 minutes. Remove from heat, and stir in ginger.
Combine 1/4 cup plus 2 tablespoons granulated sugar and zest. Whisk in yolks, cornstarch, and flour, and then the juice mixture. Return to pan. Set over medium-high heat. Cook, whisking constantly, until thickened, 2 to 3 minutes. Let mixture cool, stirring occasionally, to room temperature. Stir in yogurt.
With a mixer fitted with the whisk attachment, beat egg whites on medium speed until foamy. Add salt and cream of tartar, and beat until soft peaks form. With machine running, gradually add remaining 2 tablespoons sugar, and beat until stiff, glossy peaks form. Gently fold in grapefruit mixture in 3 additions.
Divide mixture among prepared ramekins, and place them on a rimmed baking sheet. Bake until golden and puffed, 20 to 25 minutes. Dust with confectioners' sugar. Serve immediately.