Combine the cream and sugar in a saucepan and cook, stirring, until the sugar is dissolved.
Soften the gelatin in the cold water and stir it into the cream mixture.
When the gelatin is dissolved, beat in the sour cream with a wire whisk or a rotary beater. Stir only until the mixture is thoroughly blended and smooth. Flavor with cognac or kirsch, if desired, and pour into a lightly oiled two-cup mold.
Chill for three to four hours, until firm. Unmold. Serve with sweetened crushed strawberries or raspberries.