Heat the oven to 175C. Line the base of a 23cm springform cake tin with baking paper. Lightly butter the sides, sprinkle with flour and shake out the excess.
Trim the rhubarb into lengths that will fit into the lined tin, then cut each piece lengthways in half (or into four), depending on the rhubarb’s thickness, so you end up with roughly 1cm-wide lengths.
Put a large saute pan on a medium-high heat and add the sugar, butter, pureed raspberries and salt. Cook, stirring often, until the butter melts and the mixture starts to thicken – about six minutes – then add the rhubarb and cook, turning gently now and then, for about four minutes, until it has softened slightly but still retains its shape. Gently lift out the rhubarb and transfer to a plate. Continue cooking the liquid on a medium-high heat for another five or six minutes, until you end up with a thick, jam-like caramel. Pour this into the lined tin, then carefully arrange the rhubarb on top, the stalks all facing the same direction.
Make the sponge by putting the sugar in the bowl of a free-standing mixer with the paddle attachment in place. Add the cubed butter and vanilla paste, and beat on a high speed for two minutes, until pale and airy. Add the eggs one by one, until combined, scraping down the sides of the bowl as you go, then reduce the speed to low and add the flour, baking powder, ground ginger, lemon zest, yoghurt and salt. Mix until smooth – about a minute – then gently fold in the stem ginger. Spoon the dough in dollops all over the rhubarb and gently smooth out with the back of a spoon to cover.
Bake for 50 minutes, or until a skewer comes out clean from the centre of the cake, then remove from the oven and leave to cool for 20 minutes. Gently flip the cake on to a plate, release from its tin and leave to cool completely – about an hour. Mix the cream and yoghurt in a bowl and serve alongside the cake.
