Set the oven to 180°C/350°F/Gas Mark 4.
For the topping: Melt the butter in a frying pan and add the sugar and 2tbsp water. Stir until the sugar has melted, then simmer gently until it starts to turn a pale golden colour. Add the rhubarb and stir well to coat. Remove pan from the heat. Spoon the rhubarb in a single layer over the base of the cake tin.
To make the cake: Sift the flour, ground ginger, baking powder, bicarbonate of soda and salt into a bowl. Stir in the sugar and peppercorns. Whisk together the buttermilk, eggs and oil and stir into the flour mixture. Pour over the rhubarb.
Bake the cake in the centre of the oven for 35-45 mins, or until it springs back when pressed in the centre. Remove cake from the oven and leave it to cool in the tin for about 10 mins. To serve, invert the cake on to a serving plate, remove tin and peel away lining paper. Serve warm with cream or custard.