In a medium saucepan, combine the diced rhubarb, superfine sugar, and vanilla extract. Cook over medium heat, covered, for about 15 minutes, stirring occasionally until the rhubarb softens but retains its shape. Once ready, remove from heat and let the mixture cool while you prepare the crust.

Preheat your oven to 200°C (400°F). Using a food processor, blend the rolled oats until slightly fine. In a medium bowl, combine the blended oats with all-purpose flour, brown sugar, ground cardamom, and baking soda. Mix well to incorporate all the dry ingredients.

Cut the softened unsalted butter into the dry ingredients until the mixture resembles coarse crumbs. Incorporate the egg to bind the ingredients and continue mixing until the dough takes on a wet crumble texture.

Grease a 20 cm (8 inch) round cake tin and press two-thirds of the dough into the bottom and up the sides. Spoon the cooled rhubarb compote evenly over the crust.

Sprinkle the remaining crumble mixture on top to form a delicious layer. Place the cake in the preheated oven and bake for about 25 minutes or until the top turns golden brown.

Once baked, remove the cake from the oven and allow it to cool in the tin for 10 minutes before transferring to a wire rack to cool completely.

Slice the cooled cake and serve warm or at room temperature, optionally topping each slice with whipped cream or a scoop of vanilla ice cream.