Prepare the Rhubarb Compote: Line a square cake tin with parchment paper. In a medium saucepan, combine chopped rhubarb, superfine sugar, and vanilla extract. Cook for about 15 minutes until it forms a compote.

Make the Crust Mixture: In a mixing bowl, blend the rolled oats, then add flour, baking soda, caster sugar, and brown sugar. Cut in softened butter until crumbly, then mix in the egg until well combined.

Assemble and Bake: Preheat oven to 200°C (400°F). Press two-thirds of the crust mixture into the tin, pour the rhubarb compote over, and crumble the remaining mixture on top. Bake for 25 minutes.

Cool and Serve: Once baked, cool in the tin for 15-20 minutes, slice, and serve warm with whipped cream or yogurt.