In a medium-size bowl, stir Jello powder into boiling water and keep stirring until it is completely dissolved.
Fill a 1 cup measure with ice. Fill the remaining space in the measure with water to make 1 cup ice water. If using GramZero brand jello, don't use ice water, just the same amount of cold water.
Stir the ice water into the Jello and keep stirring until cooled and the ice cubes are mostly melted. Remove any unmelted ice cubes. The Jello will soft set while you do the next step.
In a large bowl, use an electric mixer to whip together sweetener and heavy whipping cream. Whip until stiff peaks form.
Use the electric mixer to whip the soft set Jello into the whipped cream. Mix until fully combined.
Pour the Jello whip mixture into 4 custard cups or 1 medium-sized bowl and refrigerate for 2 hours.