Measure out the heavy cream and set it aside. Separate the egg yolks into a clean bowl.
In a saucepan, combine remaining heavy cream with keto sweetener and heat over medium heat, stirring constantly until the mixture just begins to steam—do not let it boil. Remove from heat and prepare to temper the egg yolks.
Whisk the egg yolks in a bowl until smooth. Gradually add a small amount of hot cream mixture into the yolks while whisking continuously to temper them. Pour the tempered yolks back into the saucepan with the remaining cream.
Return the saucepan to medium heat, stirring constantly with a spatula or spoon. Cook until the mixture thickens slightly and coats the back of the spatula—about 5-7 minutes. Use a thermometer to ensure the temperature reaches 170°F (77°C), then remove from heat.
Stir in vanilla extract. Strain the custard through a fine sieve into a clean bowl to remove any cooked egg bits. Allow it to cool to room temperature, then cover and refrigerate until thoroughly chilled, about 2 hours or overnight.
Once chilled, pour the custard into a freezer-safe container. Use a spoon or spatula to stir briefly, then place in the freezer. Every 30 minutes, stir vigorously to break up ice crystals and promote a smooth, creamy texture. Repeat 3-4 times until fully frozen and creamy. Serve directly from the freezer for best velvety consistency.
