Prepare the cocoa mixture: In a medium-sized bowl or container, add 1 ½ cups of skim milk. Then, whisk in the cocoa powder, Splenda, and cinnamon. Continue whisking until the mixture is smooth and well-combined, ensuring that the cocoa dissolves completely.
Add the oats: Once the milk mixture is homogeneous, stir in the oats. Make sure the oats are fully submerged in the cocoa mixture.
Chill: Cover the bowl or container with a lid or plastic wrap. Let the oats sit in the refrigerator for at least 4 hours or, ideally, overnight to absorb the liquid and soften.
Top and serve: Before serving, sprinkle with chocolate chips and, if desired, a pinch of salt. Optionally, add more cinnamon or a drizzle of honey for extra flavor.