Step 1In a heavy medium saucepan, bring the milk and vanilla bean to a simmer. Meanwhile, in a bowl, beat the yolks, sugar and cornstarch to blend.
Step 2Gradually whisk the hot milk into the egg mixture. Return the mixture to the saucepan and whisk over medium heat until it thickens and boils gently for 1 minute. Strain the pastry cream into a small bowl set over a bowl of ice water, gently stirring until the pastry cream cools. Cover and refrigerate until chilled, about 1 hour.
Step 3Stir the pastry cream to loosen, and fold in the yogurt. The pastry cream can be prepared up to 3 days in advance; cover and refrigerate.
Step 1Heat the oven to 350 degrees. In a small bowl, whisk together the flour, sugar and sea salt. Add the butter pieces and rub the mixture between your fingertips to form pea-sized crumbles. Transfer the crumbles to a heavy, small, rimmed baking sheet and spread to form a single layer. Bake, stirring once or twice, until the crumbles are lightly golden, about 15 minutes. Cool. The crumbles can be made up to 3 days in advance; store in an airtight container.
Step 1Supreme the pomelos: Using a very sharp knife, cut off the top and the bottom of the fruit, so it will sit flat on the cutting board. Starting where you see the pomelo separate from the white pith, cut away one section of peel and pith, following the line of the fruit. This will expose the underlying fruit. Continue cutting away sections of the peel and pith until only fruit remains. When you're done, go back over the fruit, removing any traces of pith. Working over a bowl, separate each segment, breaking each segment into bite-sized pieces if desired. Strain the pomelos, reserving the juice.
Step 2In a heavy, small saucepan, combine 1/4 cup of the reserved juice, or water (if pomelos are not juicy) with the sugar and ginger. Stir the mixture over medium heat until the sugar dissolves and the syrup just boils, about 2 minutes. Cool the syrup slightly and pour over the pomelos; stir gently. (Compote can be prepared 1 day ahead; cover and refrigerate.)
Step 3To assemble, divide the pomelos among four bowls, topping with the yogurt and crumbles. Serve immediately.