What do you add to whipping cream to keep it stiff?

Unflavored Gelatin is a good stabilizer for whipped cream. Use 1 tsp. unflavored gelatin, 1 c. heavy whipping cream, 4 tsp. cold water, 1/4 c. powdered sugar, 1/4 tsp. vanilla. Combine gelatin and water. Let stand until thick. Then place over low heat, stirring constantly, until gelatin dissolves, which takes about 3 minutes. Whip cream with 1/4 cup confectioners' sugar and 1/4 teaspoon vanilla, just until it is slightly thickened. While beating slowly, gradually add gelatin to whipped cream mixture. Whip at high speed until stiff.