Combine chocolate, 1/4 cup of milk and sugar replacement in top of double boiler. (Chill remaining milk in freezer.)
Cook and stir over simmering water until chocolate melts. Pour small amount of hot chocolate mixture over egg yolks and beat well. Pour egg mixture into chocolate mixture on top of double boiler. Stir in salt.
Cook and stir until mixture thickens. Cool completely. Scrape cold or slightly frozen milk into mixing bowl and beat until very stiff. Fold chocolate mixture into stiffly beaten milk. Fold in vanilla and banana slices.
Spoon into mold, freezer tray or individual cups and freeze until firm.