Prepare Rhubarb Compote: Line an 8-inch round cake tin with parchment paper. In a saucepan over medium heat, combine chopped rhubarb, superfine sugar, and vanilla extract. Stir for about 15 minutes until the rhubarb is soft but holds shape. Cool completely.

Make Cake Crumb: Preheat oven to 400°F (200°C). In a blender, pulse rolled oats until slightly finer. In a mixing bowl, combine this with flour, brown sugar, cardamom, and baking soda. Cut in softened butter until crumbly, then mix in egg until wet crumbs form.

Assemble and Bake: Press two-thirds of crumb mixture into the tin to form a crust. Spread cooled rhubarb compote over the crust, then sprinkle remaining crumb on top. Bake for about 25 minutes until golden brown. Cool before removing from tin.