Preheat oven to 350 degrees F. Cream butter, sugar, and eggs. Fold in molasses. Add flour, buttermilk and spices with soda alternately, stirring well after each addition. Put into an 8x8-inch pan and bake for 25 minutes.
In a saucepan, combine sugar and cornstarch. Slowly add 1 cup of water. Cook over low heat for 5 minutes or until thickened. Stir in butter, lemon zest, lemon juice, and salt. Serve cold or warm. If serving cold, fold in whipped cream.