Photograph: Maja Smend
Preheat the oven to 180°C, fan 160°C, gas 4. Spray a little oil or lightly grease a 20cm square baking tin and line with baking paper so it overhangs the tin, making it easier to remove the cake.
Put all of the ingredients for the sponge into a large bowl with a large pinch of salt. Mix together until well combined. Spoon the mixture into the tin and smooth the top with the back of a spoon. Bake for 30-35 minutes or until the cake is springy to the touch and a skewer inserted into the centre comes out clean. Leave to cool completely in the tin.
Reserving a little stem ginger to decorate, mix all of the frosting ingredients in a bowl until smooth. Cover and chill until ready to use.
Remove the cooled cake from the tin, peel the paper off and transfer to a serving plate. Spread the frosting evenly over the cake and sprinkle over the reserved stem ginger.
Get ahead: make the cake up to 3 days ahead. Ice when ready to serve. The uniced cake can also be frozen.