Beat butter and sugar together in a large bowl until pale and very fluffy. Beat in 1 egg yolk at a time, mixing well after each addition, gradually adding lemon rind and juice until well mixed. Crumble lemon coolers and add to mixture. Don't worry if the mixture curdles at this stage.

Sift flour and stir into lemon mixture until very well mixed. Gradually stir in milk.

In a separate bowl whisk egg whites until stiff peaks form, then lightly but thoroughly, fold into the lemon mixture in three batches. Carefully pour mixture into prepared soufflé dish or cake tin, and set in the roasting pan. Pour water into the roasting tin until it comes halfway up the sides of the soufflé dish. Bake in center of oven for 45minutes until golden brown on top. Serve at once.