In a large soup pot, melt together the butter and marshmallows.
Once the marshmallows are completely melted, remove the pot from the heat and add the vanilla.
Pour in the cereals and mix until totally combined.
Pour the mixture into a parchment-lined 8" square baking pan and using a sturdy spoon or your hands, press down the mixture firmly into the pan.
Let set overnight (or at least 2 hours).
Drizzle with melted peanut butter and sprinkle with cinnamon.