In a small bowl mix nutritional yeast, cornstarch, xanthan gum, paprika, and turmeric and set aside; Mix the milk and water in a cup and set aside.
In a small saucepan melt the vegan butter over low heat. Add the dry mixture and stir until smooth.
Add milk and water. Increase to medium heat and cook, stirring occasionally, until the sauce begins to bubble slightly. The yellow color of the yolk will develop while cooking.
Lower the heat again. Cook for another 2-3 minutes. Stir occasionally. Remove from heat. At this point you can add Annatto oil to deepen the color - it is recommended to add it gradually until you get the desired hue.
Remove from heat, add the black salt and stir.
The yolk sauce keeps in the fridge for about a week and can also be frozen. (See note at the bottom of the recipe)