Chop rhubarb, combine with sugar and vanilla bean seeds, let sit for 60 minutes, preheat oven to 350°F (175°C), spread mixture on a baking sheet, roast for 25 minutes.

Combine flour, sugar, cold butter, and cinnamon in a mixing bowl until it resembles coarse crumbs. Refrigerate for 15 minutes, spread on a baking sheet, bake for 12-15 minutes until golden brown, cool completely.

Whisk egg yolks and sugar until smooth. In a saucepan, combine heavy cream, milk, and salt, bring to a gentle simmer. Temper egg yolk mixture with hot cream, return to saucepan, stir for about 5 minutes, strain into another bowl and cool over an ice bath for at least 2 hours.

Stir in crème fraîche, fold in half of the roasted rhubarb and half of the crumble. Pour into ice cream maker and churn according to manufacturer's instructions for about 25 minutes.

Transfer churned ice cream into an airtight container, layer with remaining rhubarb and crumble, cover with plastic wrap and seal. Freeze for at least 4 hours or until firm enough to scoop.

Scoop ice cream into bowls or cones, serve with pound cake or caramel sauce if desired.