Heat the oven to 180C/fan 160C/gas 4. Butter two 20cm sandwich tins and line with baking paper. Beat together half (175g) the butter and half the sugar with an electric whisk until pale, then add 3 of the eggs, one at a time, beating between each addition. Mix in half of the vanilla, flour and spices, along with a small pinch of salt. Fold in half the nuts and divide the batter between the tins, smoothing the tops with the back of a spoon. Bake for 25-30 minutes or until well risen and a skewer inserted into the middles comes out clean. Cool in the tins for 5 minutes, then transfer to a wire rack to cool completely. Clean the tins and repeat this step with the remaining sponge ingredients to make a total of four sponges.