Combine 1.5 cups sugar, coffee and corn syrup in saucepan at least 13 cm deep. Bring mixture to boil and cook to 150ºC on candy thermometer or hard-crack stage (small amount of mixture dropped into cold water will break with brittle snap). Remove syrup from heat, immediately add soda and stir vigorously just until mixture thickens and pulls away from sides of pan. (Mixture will foam rapidly when soda is added. Do not destroy foam by excessive beating.) Immediately pour foamy mass into ungreased 23 cm square metal pan (do not spread or stir). Let stand without moving until cool. When ready to garnish cake, knock hardened mixture out of pan and crush between sheets of waxed paper with rolling pin to form coarse crumbs. Split cake into 4 equal layers. Combine cream, remaining 2 tablespoons sugar and vanilla in chilled bowl and beat until stiff. Spread about half of cream between layers and remainder over top and sides of cake. Sprinkle generously with crushed topping. Refrigerate until ready to serve.