Rhubarb, vanilla and soured cream crumb cake

Method

1. For the crumbs, melt the butter in a small pan over a low heat. Remove from the heat, add both types of sugar and stir constantly until dissolved into a smooth, toffee-like sauce. Stir in the vanilla bean paste, pour into a small mixing bowl, then stir in the flour and salt to make a stiff, biscuit-like dough. Press the mixture firmly into the bottom of the bowl and set aside to go cold.
2. Preheat the oven to 180°C/fan 160°C/gas 4. Grease and line a shallow 20cm square, 5cm deep loose-bottomed cake tin with baking paper. (Or use a 23cm round cake tin to replicate our cake on the cover.) Wipe the rhubarb stalks clean, trim off the ends, cut the remainder into 2.5cm pieces, then put into a mixing bowl. You should be left with about 225g rhubarb. Mix the sugar with the flour, stir into the rhubarb and set aside for 15 minutes or so, stirring now and then, until the flour and sugar mixture has become moist and clings to the fruit. Set aside.
3. For the cake, stir the soured cream, egg, egg yolk and vanilla in a small bowl. Sift the flour, sugar, bicarbonate of soda, baking powder and salt into another bowl. Add the butter and mix using a hand-held electric mixer until the mixture looks like fine breadcrumbs. Stir in the nuts, then gradually beat in the soured cream mixture until smooth.
4. Spread the cake mixture into the base of the prepared tin, then scatter over the rhubarb mixture. Break the crumb mixture into hazelnut-size pieces (or smaller if you prefer) and scatter over the cake. Bake for 40-45 minutes until golden and a skewer inserted into the cake comes away clean (don’t forget – if you hit a piece of rhubarb it will still be moist). Leave to cool for 10 minutes, then dust with icing sugar. Cut into 9 squares and serve either warm with crème anglaise, or cold.