Rhubarb & custard crumble cake

Serve Mum the mother of all desserts that she deserves with a slice (or two) of this sweet, decadent last course

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125g butter softened, plus extra for the tin 400g rhubarb chopped into 1cm chunks 200g golden caster sugar 150g self-raising flour 50g quick-cook polenta 3 tbsp custard powder 1/2 tsp baking powder 2 eggs 1 tsp vanilla extract 125ml soured cream icing sugar for dusting double cream to serve (optional) FOR THE CRUMBLE TOPPING 100g golden caster sugar 60g plain flour 2 tbsp custard powder 1 tbsp quick-cook polenta 50g butter, cubed