Stir the vanilla and brown sugar into the cold water until dissolved.
Add the oats, salt, and cold water mixture to a blender. Blend on high for 30 seconds.
Pour the blended oat mixture through a strainer like a nut bag, cheese cloth, metal coffee filter, or a double layer of metal sieves to filter out the oat pieces.
Chill it completely (preferably overnight), then shake very well before serving. Keep it refrigerated for up to four days.
*I think adding a little vanilla really rounds out the flavor so it's more interesting and enjoyable. You can skip it, but I really feel like the vanilla takes this recipe from good to GREAT.**You can use any type of sweetener that dissolves easily in cold water.I used brown sugar, but maple syrup is even more delicious (just more expensive). Other options are white sugar, honey, agave, stevia, monk fruit, or any other sweetener that dissolves easily in cold water.***I tested both old-fashioned rolled oats and quick oats for this recipe, and old-fashioned oats were the clear winner. Old-fashioned rolled oats break down just enough when blended to yield creamy milk that is not too thick or slimy. Quick oats are so powdery to begin with that they completely disintegrate in the water and make a thick, sludge-like milk. There is no nutritional information for this recipe because nutritional calculators can not account for the oats that are discarded in the straining process.