1. Preheat oven to 350 degrees Fahrenheit. Grease and line a seven- by 11-inch tin pan with wax paper.
2. Dissolve coffee in boiling water.
3. Cream margarine, peanut butter, sugar, and coffee until light.
4. In a separate bowl, sift together flour and cornstarch.
5. Mix flour mixture into creamed mixture to make smooth dough. Press dough into the bottom of the pan and prick all over with a fork.
6. Bake for 20 minutes. Meanwhile prepare topping.