In a small saucepan, melt butter and brown sugar.
Cook over medium-low heat, stirring often until melted.
Add vanilla extract and heavy cream, stirring well.
Cook for 2-3 minutes, stirring frequently.
Let the mixture bubble slightly. Sugar should dissolve.
Remove from heat. Measure ½ cup of sauce into a bowl.
Store the rest of the sauce in an airtight container.
In a mixing bowl, whisk heavy cream into the sauce.
Add whole milk and whisk until combined.
Pour the mixture into a Ninja Creami pint.
Close the pint lid and freeze for at least 24 hours.
Remove the pint from the freezer. Take off the lid.
Place the pint into the outer bowl of the Ninja Creami.
Close the outer bowl lid and secure it in the machine.
Use the Ice Cream button to process the ice cream.
When done, check the ice cream’s texture.
If crumbly, re-spin it using the Re-spin function.
If still crumbly, add 1-2 tablespoons of milk and re-spin.
The ice cream should have a soft-serve texture.
Warm reserved butterscotch sauce in the microwave for 10 seconds.
Drizzle warm sauce over each serving of ice cream.