Step 1

Preheat oven to 190C. Grease an 18cm x 27cm slice tin and line with baking paper.

Step 2

Cream butter and icing sugar until pale. Add zest, lemon juice and sour cream and beat well. Add flour and beat until just combined. Spoon mixture into prepared tin and press into an even layer.

Step 3

Bake for 15 mins. Remove from the oven and allow to cool in the tin.

Step 4

Make icing by beating butter and sour cream until smooth. Add icing sugar gradually until it has fully incorporated, then add lemon juice. Beat at high speed for 4 minutes until icing is pale and thick. Spread over the slice base in the tin and refrigerate for up to an hour to set icing. Slice into squares and serve.