Heat oven to 180C/fan 160C/gas 4 and make sure there’s a shelf ready in the middle. Butter and line the bases of 2 x 20cm round sandwich tins with baking paper.
Using electric beaters, beat together all the cake ingredients with a pinch of salt until smooth, then spoon into the tins and level the tops. Bake for 30 mins or until golden and springy. Don’t open the oven before 25 mins cooking time has passed.
When they’re ready, cool the sponges for a few mins, then tip out of the tins and cool completely on a wire rack. Make sure the top of one of the cakes is facing up as you’ll want a smooth surface for the icing later on.
When cool, put one sponge on a serving plate, then spread with jam. Sandwich the second sponge on top. Sieve the icing sugar into a large bowl. Add the water or lemon juice, then stir until smooth and thick. Spread evenly over the top and let it dribble over the sides. Scatter with the nuts and leave to set for a few mins before cutting.