In a medium heatproof bowl, whisk the yolks just to break them up and whisk in ¼ cup sugar. Set aside.
In a heavy non-reactive saucepan, combine cream, milk, salt and ¼ cup sugar.
Put the pan over medium heat and let the mixture boil gently to bubbling just around the edges (gentle simmer). Reduce the heat to low.
Carefully measure out ½ cup of hot cream mixture. Whisk in the hot cream mixture while whisking the eggs constantly. Continue tempering the eggs by adding another ½ cup of hot cream to the bowl with the yolks.
Pour the cream-egg mixture back to the saucepan and cook over medium-low heat, stirring constantly until it is thickened and coats the back of a spatula, about 12-15 minutes.
Strain the base through a fine-mesh strainer into a clean container.
Pour the mixture into a 1-gallon Ziplock freezer bag and submerge the sealed bag in an ice bath until cold, about 30 minutes. Refrigerate the ice cream base for at least 2 hours or overnight. I like to refrigerate the base overnight for the most flavor.
When ready to churn the ice cream, whisk the creme fraiche, fold in half of the roasted rhubarb and 1/2 cup of crumble into the ice cream base.
Pour the ice cream base into the frozen canister of your ice cream machine and follow the manufacturer’s instructions.
Spin until thick and creamy about 25-30 minutes.
Pack the ice cream into a storage container, press a sheet of parchment directly against the surface and seal with an airtight lid. Freeze in the coldest part of your freezer until firm, at least 4 hours.
When ready to serve, scoop ice cream into bowls. Top with remaining roasted rhubarb and extra crumble. (optional)