Grease an 8x8x2 with cooking spray and set aside. In a large pot on low, heat the butter until glossy and melted. Add the honey, peanut butter, and cinnamon and stir until incorporated. Add the marshmallows and stir constantly. This will take a couple minutes, and it will seem like they will never melt, but it is important to keep the heat low to get that perfect, gooey texture. Once the marshmallows have melted entirely and the mixture is thick, smooth, and golden, turn off the heat. Stir in Cheerios and pour the mixture into the prepared pan. Refrigerate for 30-45 minutes, then cut into 9 squares. Serve.