Preheat oven to 375°. Put plump raisins in boiling water for 5 minutes. Drain on paper towels. Cream room temperature butter and gradually add sugar. Beat until light. Add beaten egg. Stir in raisins and lemon juice. Fill pastry-lined tart tins with a teaspoon of the mixture (about 1/3 full). Bake for 15 to 20 minutes.