1. Combine brown sugar, instant coffee, water, corn syrup and butter or margarine in a small saucepan. Heat slowly, stirring constantly, until bubbly; then cook and stir 3 minutes; remove from heat.
2. Stir in semisweet chocolate pieces and vanilla until chocolate melts and sauce is smooth. Serve warm. To reheat: Place in top of a double boiler over simmering water.