Preheat your oven to 350°F (175°C). Toss chopped rhubarb with sugar and vanilla bean seeds, let sit for 60 minutes, then spread on a baking sheet and roast for 25 minutes.

Combine flour, sugar, cold butter, and cinnamon in a bowl. Mix until it resembles coarse crumbs. Chill for 15 minutes, then bake at 350°F (175°C) for 12-15 minutes until golden.

Whisk egg yolks and sugar in a saucepan until pale. Heat heavy cream and milk in another pan until boiling, then gradually whisk it into the egg yolk mixture. Cook until thickened, about 5-7 minutes.

Strain the thickened mixture through a fine mesh sieve into a bowl. Cool to room temperature, stir in crème fraîche, cover, and refrigerate for at least 2 hours.

Pour the chilled ice cream base into your ice cream maker. Churn according to the manufacturer's instructions for 20-25 minutes, folding in half of the roasted rhubarb and crumble before finishing.

Transfer churned ice cream to a freezer-safe container, pressing parchment paper against the surface. Freeze for at least 4 hours.

Scoop the ice cream into bowls and top with remaining roasted rhubarb and crumble.