Line a 20 cm (8-inch) round cake tin with parchment paper. In a medium saucepan, combine fresh rhubarb, superfine sugar, and vanilla extract. Cook over medium heat for about 15 minutes, stirring occasionally, until rhubarb is soft but holds shape. Set aside to cool.

Preheat your oven to 200°C (400°F). Blend rolled oats in a food processor until slightly finer. In a mixing bowl, combine oats, plain flour, brown sugar, ground cardamom, and baking soda. Cut in softened unsalted butter until coarse crumbs form, then mix in the egg to form wet clumps.

Press about two-thirds of the dough into the prepared cake tin to cover the base and sides evenly. Spoon the cooled rhubarb compote onto the crust and spread it evenly. Crumble the remaining dough over the top. Bake for approximately 25 minutes, or until golden and crisp.

Once baked, let the cake cool in the tin for a few minutes. Transfer it to a wire rack to cool completely. Slice and serve with whipped cream for an indulgent touch.