Place whipping cream in a small bowl. Add 3 tablespoons chocolate sauce, coffee granules, and vanilla, stirring until granules dissolve. Beat whipping cream mixture at medium speed of an electric mixer until foamy; gradually add powdered sugar, beating until soft peaks form.
Place 9 graham cracker squares in an 8" square pan. Spread one-third of cream mixture over crackers. Repeat layers twice with remaining ingredients. Cover and chill at least 8 hours. Garnish, if desired.