I expect grapefruit desserts to be either wet and syrupy (with rings of citrus I'm never quite sure how to navigate), or muted and shy (grapefruit in name, not in essence): But here, the grapefruit is serious and confident and leaning in. It's helped, not hindered by the airiness of the custardy dessert.
The smartest part of this recipe is in the first step: Fresh grapefruit juice is simmered on the stovetop until it reduces from 3 cups to 1/2. You sharpen the tart, concentrate the sweet, eliminate the excess liquid. The resulting flavor is so pleasingly bitter, you'll swear someone tipped a bit of Aperol into the batter (which is not a bad idea).
Preheat the oven to 350° F. Butter a 10-inch round baking dish (I've also used a 9-inch baking dish—turned out just fine).
Pour the grapefruit juice into a small saucepan and simmer, uncovered, over medium heat until it has reduced to 1/2 cup, 35 to 40 minutes. Let cool completely (you can stick it in the freezer or submerge a bowl filled with ice.
Make the vanilla sugar: Split the 1/2 vanilla bean, then use the dull side of a butter knife to scrape the seeds out into the sugar bowl. Mix with a spoon or your fingers until the sugar is perfumed with vanilla.
As the grapefruit juice simmers, whisk 1 cup of vanilla sugar with the flour and salt in a large bowl (reserve extra vanilla sugar for the cake accoutrements!).
In a separate bowl, whisk the cooled grapefruit juice, yogurt, crème fraîche, zest, and egg yolks. Whisk this mixture into the dry ingredients.
In the bowl of a stand mixer, whisk the egg whites until soft peaks. Gently fold the egg whites into the grapefruit mixture. Do your best not to deflate the egg whites.
Pour into the prepared baking dish and bake until the pudding is set but a bit jiggly in the middle and the top is golden, 25 to 30 minutes. Let rest for 5 minutes before serving warm. (It's okay if the top of the cake cracks as it cools.)
Serve topped with vanilla wafer crumbs and a dollop of bitters-spiked crème fraîche.
For getting fancy:
To make the Nilla Wafer crumbs: In a medium sauté pan, melt the butter over medium-low heat. Add the cookie crumbs and stir to coat. Once the crumbs are coated in butter, sprinkle the vanilla sugar over top and stir to combine. Cook, stirring, for 5 to 7 minutes, until the cookie crumbs are crisp. Let cool completely so that they're as crunchy as possible.
To make the bitters-spiked crème fraîche: Scoop crème fraîche into a serving bowl (I used about 1/3 cup) and add a few dashes of bitters. Stir to combine. Add natural sugar to taste.
A (former) student of English, a lover of raisins, a user of comma splices. My spirit animal is an eggplant. I'm probably the person who picked all of the cookie dough out of the cookie dough ice cream. For that, I'm sorry.