Put the rhubarb and sugar into a medium saucepan over medium-high heat. Give it a mix around as it heats and the liquid seeps out of the rhubarb. After 10 minutes it will have softened, add the vanilla and maple syrup and reduce the heat to low. Stir frequently as it will bubble a lot. Use a lid if you like, to stop it spitting everywhere. It should be ready in 20 minutes.
Set aside to cool completely as you make the rest of the cake.
Preheat the oven to 200°C and grease a 25 cm tart tin with a removable base.
Put the oats, flour, sugars, cardamom, ginger and bicarb into the bowl of a food processor. Pulse it a couple of times to mix it all together.
Drop tablespoons of the butter into the processor and pulse until it has incorporated through the dry ingredients. Add the beaten egg and pulse until it just mixes through the other ingredients. If the egg sticks to the bottom, you may need to scrape everything out into a large bowl or the bench top and mix it together by hand. Just don’t over-mix it.
Alternatively, do it by hand in a large mixing bowl. Rub the butter through the dry ingredients, mix through the egg and continue with the next step.
Cut one third of the dough from the mass and set it aside. Press the remaining dough into the greased tin (wet your hands, if necessary), evenly covering the bottom and about 3 cm up the sides.
Spread the cooled rhubarb jam into the prepared tin, smooth the top and then break up small pieces of the reserved dough all over the rhubarb jam.
Bake in the oven for 20-25 minutes, or until golden. Cool in the tin completely before removing.
Dust with icing sugar, slice and serve with double cream.