Soak the yellow soybeans for 24 to 36 hours in a generous amount of water. Strain, rinse, and change the water about half-way through this soaking period (or every 12 hours if soaking longer). Strain and rinse the soybeans before using.
In a food processor add the soaked soybeans and ½ a cup of water. Puree the beans and water at the highest setting (liquefy and/or turbo) for 90 seconds. Move the raw soy pulp into the cooker.
Add 5 cups of water and mix everything together well – ensure that the contents of the cooker do not exceed more than half the pot capacity.
Bring the uncovered cooker to a boil stirring occasionally. You will know the soy is boiling when the foam increases in volume quickly.
Remove the foam from the top and discard, then wipe down the sides of the pot with a spatula and give the contents one last stir. Close the lid and set the valve to the pressure position.
Pressure cook on HIGH for 9 minutes if using an electric unit, or 7 minutes if using a stovetop unit.
In the meantime, in a wide-mouth pitcher or bowl, add the sugar, vanilla bean, and salt (if using) and place a fine-mesh strainer on top.
When time is up, open the pressure cooker using the natural release method.
Carefully pour the hot contents of the cooker through the fine-mesh strainer into the pitcher, pushing down on the pulp with the spatula to wring out any remaining milk from the pulp.
Mix the contents in the pitcher well and let cool and then remove and rinse off the vanilla bean to use for your next batch.
Cover tightly and refrigerate – keeps for 3-4 days in the refrigerator. Shake, or mix, before using.