Combine the dry ingredients and sift into the egg mixture. Fold in until incorporated. Pour the batter into the prepared cake tin and spread out evenly. Spread the pastry cream ( recipe here ) evenly on top of the batter. Remove the rhubarb from the syrup and scatter over the cake. Reserve the syrup for drizzling over the cake once it’s cooked. Bake for about 1 hour in the oven (lowering the heat if the top gets too dark). A skewer inserted into the middle of the cake should come out clean, although the pastry cream will remain a bit wet. Remove from the oven and allow to cool slightly before drizzling with leftover rhubarb syrup and cutting into slices to serve.