Begin by greasing your slow cooker with nonstick spray. Then, add all ingredients to the slow cooker and whisk until the cocoa powder is mostly dissolved.
Cover and cook on high for 2-4 hours, stirring every 30-60 minutes to prevent sticking. Once the oats have absorbed most of the liquid and become chewy, they're ready.
Serve your hot cocoa steel-cut oats topped with mini marshmallows or chocolate chips, and savor the cozy flavors!