In a saucepan, heat 1 1/2 cups of the heavy cream with 1/2 cup of the sweetener. Bring to a simmer, but do not let it come to a boil
In a medium bowl, mix the rest of the heavy cream and sweetener with the egg yolks, salt, cinnamon, ginger, and pumpkin spice until the sweetener has dissolved
Remove the cream mixture from the heat, and pour a small amount (~1/2 cup) into the egg yolk mixture. Whisk rapidly to prevent the egg from cooking. This should raise the temperature of the egg mixture
Combine the warmed egg mixture with the cream mixture in the pan, and then return to the heat. simmer over medium heat for 4-6 minutes, stirring constantly
When the mixture has thickened enough to coat the back of the spoon, strain it though a fine-mesh sieve and transfer it to a plastic or glass bowl and place in a larger bowl of ice water to cool while stirring occasionally
Once cooled, cover the custard with plastic wrap, allowing the plastic to touch the surface of the custard.
Refrigerate for at least 3 hours, up to overnight.
After it has sufficiently cooled, freeze the custard using an ice cream maker, or freeze manually by placing it in the freezer and stirring the mixture every 2-3 hours until frozen.