Preheat your oven to 200°C (400°F). Line a 20 cm (8 inch) round cake tin with parchment paper.

In a medium saucepan, combine chopped fresh rhubarb, superfine sugar, and vanilla extract. Cook over medium heat for about 15 minutes, stirring occasionally.

Blend rolled oats in a food processor for a finer consistency. In a mixing bowl, combine blended oats with plain flour, brown sugar, ground cardamom, and baking soda.

Cut softened unsalted butter into the dry mixture until it resembles coarse crumbs. Add a room-temperature egg and stir until it forms a wet crumble.

Press two-thirds of the crumble mixture into the bottom and up the sides of the prepared cake tin. Spoon the rhubarb compote over it and crumble the remaining one-third of the dough on top.

Bake for approximately 25 minutes in the preheated oven until golden brown.

Remove from oven and let cool in the tin for about 10 minutes before lifting it out. Allow to cool completely on a wire rack.