In a small bowl, stir together matcha powder and about 1 tablespoon of warm water until dissolved.
Add soy milk, oat milk, dissolved matcha powder, soy lecithin and soy protein isolate (if using) to a mason jar and shake to combine. Alternatively, you can stir all ingredients together in a bowl. If you're not using right away, keep in an airtight container in the fridge (you can use this nondairy matcha sweet cream in drinks as is if you want).
Turn mixture into cold foam right before using. You can either use an electric frother, hand mixer, or blender. Just add about ⅓ of the sweet cream and froth, mix, or blend until the mixture turns into a foam (I usually froth half of the batch at a time in my electric frother). Alternatively, you can shake the mixture in a mason jar until it's frothy.