Shortcrust pastry dough
Mix the flour, almonds and sugar in a bowl. Add the butter, rub by hand or pulse in a food processor until you have an even, crumbly mixture. Transfer 3/4 of the dough to the prepared tin and press down firmly by hand. If the mixture is too crumbly, mix in 1-2 tbsp of water. Chill the tin with the dough base and the remainder of the dough for the crumble for approx. 30 mins.
Mix the cornflour with the water and stir until smooth, bring to the boil in a pan along with the rhubarb, sugar and vanilla, simmer for approx. 5 mins. while stirring occasionally, leave the stewed rhubarb to cool.
Spread the stewed rhubarb over the base. Top with the reserved crumble and almonds.
Bake for approx. 30 mins. in the centre of an oven preheated to 180°C.
Leave the cake to cool, cut into slices, dust with icing sugar.