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Heat oven to 180C/160C fan/gas 4. Grease and line the base of a 20cm square deep cake tin (or if using a decorative tin like ours, grease well).
In a bowl, beat the butter and sugar until fluffy. Add the rest of the ingredients and beat until combined. Spoon into the tin and smooth over the top. Bake for 40-45 mins. Cool in the tin for 10 mins, then turn out, peel off the parchment and leave to cool on a wire rack.
Sift the icing sugar into a bowl. Beat in the lemon juice to make a runny icing. Pour over the cake and leave to set.