Preheat oven to 350°F (175°C).

Butter a 10-inch round baking dish.

Pour grapefruit juice into a saucepan and simmer over medium heat until reduced to 1/2 cup (35-40 minutes).

Let the reduced grapefruit juice cool completely.

Make vanilla sugar: split vanilla bean and scrape seeds into sugar. Mix well.

Whisk vanilla sugar, flour, and salt in a large bowl.

In a separate bowl, whisk cooled grapefruit juice, yogurt, creme fraiche, zest, and egg yolks.

Whisk wet ingredients into dry ingredients.

In a stand mixer, whisk egg whites until soft peaks form.

Gently fold egg whites into the grapefruit mixture.

Pour the mixture into the prepared baking dish.

Bake until the pudding is set but slightly jiggly in the middle and the top is golden (25-30 minutes).

Let rest for 5 minutes before serving warm.

Serve topped with Nilla Wafer crumbs and bitters-spiked creme fraiche.

Make Nilla Wafer crumbs: melt butter in a pan over medium-low heat.

Add cookie crumbs and stir to coat with butter.

Sprinkle vanilla sugar over the crumbs and stir to combine.

Cook, stirring, for 5-7 minutes, until the cookie crumbs are crisp. Let cool completely.

Make bitters-spiked creme fraiche: scoop creme fraiche into a bowl and add a few dashes of bitters. Stir to combine. Add sugar to taste.