Step 1Combine sugar, coffee and corn syrup in medium saucepan at least 5 inches deep. Bring mixture to boil and cook until it reaches 310 degrees on a candy thermometer or reaches the hard-crack stage (when a small amount dropped into cold water breaks with a brittle snap).
Step 2Press baking soda through sieve to remove lumps. Remove syrup from heat. Immediately add soda and stir vigorously just until mixture thickens and pulls away from sides of pan. (Mixture foams rapidly when soda is added. Do not destroy foam by beating excessively.)
Step 3Immediately pour foamy mass, being careful to avoid spatters, into ungreased 9-inch square metal pan. (Do not spread or stir). Let stand without moving until cool, 1 hour.
Step 4When ready to garnish cake, remove from pan and crush between sheets of wax paper with rolling pin to form coarse pieces.
Step 5Frost cake with whipped cream. Cover generously and thoroughly with brittle pieces. Refrigerate until serving.