Add the rolled oats, protein powder, almond milk, cocoa powder, maple syrup, vanilla extract, and sea salt to a bowl or jar.
Stir well until everything is fully combined and the protein powder is completely dissolved.
Seal the container with a lid and place it in the refrigerator overnight.
The next morning, remove the container from the fridge and stir the oats. The oats will have absorbed much of the liquid, creating a thick and creamy texture.
If needed, add a splash of almond milk to loosen the mixture.
Divide the Brownie Batter Overnight Oats into two containers and top with chocolate chips, chopped walnuts, and peanut butter if desired.
Serve immediately and enjoy.