Combine the plant-based milk, corn starch, and sugar in a large, microwave-safe bowl.
Stir until there aren't any lumps from the corn starch.
Microwave for about a minute and stir. Repeat this until the mixture is nice and thick, usually about 3 minutes (see notes for stovetop instructions).
Use potholders to remove from the microwave. Stir in the vegan butter until incorporated.
Allow to cool before covering and placing in the fridge.
Store in an airtight container in the fridge for 7 days.
Notes
* If you're not using vanilla-flavored plant-based milk, just add 1 teaspoon vanilla to this mixture.To make this on the stove-top, add the ingredients to a saucepan and stir. Place the pan over medium heat and bring to a simmer while stirring. Simmer for 1 - 2 minutes until thickened. If lumps form, simply place it in a blender. Once the mixture cools, place it in a container and refrigerate until chilled.