In a medium saucepan, combine chopped fresh rhubarb with superfine sugar and vanilla extract over medium heat. Cook for about 15 minutes, stirring occasionally, until the rhubarb softens but retains its shape. Transfer the compote to a bowl and let it cool completely.

Preheat your oven to 200°C (400°F).

In a large mixing bowl, blend the rolled oats slightly, then combine them with all-purpose flour, both sugars, ground cardamom, and baking soda. Chop the softened unsalted butter into the dry ingredients and mix until it resembles coarse crumbs. Beat in an egg until a wet crumble forms.

Press two-thirds of the dough mixture into the bottom of a greased cake tin to form a base. Spread the cooled rhubarb compote evenly, then crumble the remaining dough mixture over the top.

Bake for about 25 minutes, or until the top is golden brown and bubbly. Check towards the end to prevent over-baking.

Let the cake cool in the tin for at least 15 minutes, then transfer to a wire rack to cool completely. Slice into squares and serve.