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.
Milk Types
Here are the types of milk I've used successfully in this recipe:
Soy milk (my favorite)
Almond Milk
Oat Milk
Coconut milk (great consistency, but it tastes very "coconutty").
If you're using a different type of plant-based milk, the consistency may be different, and cooking times may take longer.
Stovetop Method
To prepare this on the stovetop, add the ingredients to a saucepan and stir until smooth. Place the pan over medium heat and bring to a simmer, stirring occasionally. 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.
Making More Than a Single Batch
The microwave method is most effective for a single batch. If you're making more (a double or more), the stovetop will be your best bet.