In a large bowl cream together butter and sugar until light and fluffy. Beat in the egg and almond essence. Combine the flour and salt; stir into the creamed mixture until well blended. Mix in 1/2 cup of the cherries. Press the dough into an ungreased 20x30cm baking dish. Sprinkle with remaining cherries and crushed Smarties. Press in lightly.
Bake for 30 to 35 minutes in the preheated oven or until edges are lightly browned. Cool completely in the pan before icing. To make the icing, mix together the icing sugar and water until a drizzling consistency is reached.