Whisk together the sugar, cornstarch, nutritional yeast and salt.
Have your butter cubed up and ready.
In a small saucepan, heat the milk and vanilla over medium heat until barely simmering.
Add about 1/4 of the milk to the dry ingredients while whisking. Then pour that mixture back into the saucepan while also whisking.
Return to the heat and whisk constantly. When it starts to thicken, whisk vigorously for 1 to 2 minutes, making sure to get the corners. When you stop for a few seconds, you should see a few big bubbles coming to the surface and popping.
Turn off the heat and whisk in the butter until completely melted and smooth then pour the cream onto the baked base and even it out.
In the microwave: In a microwave-safe bowl, Whisk together the sugar, cornstarch, nutritional yeast and salt. Add in the vanilla then slowly add in the milk while whisking to prevent lumps. Add in the butter (melted or not) and heat the whole thing in the microwave in 30 second bursts, whisking in between, until it has bubbled, thickened and is completely smooth. Pour it over the baked base and even it out.