On medium heat, melt the butter and the marshmallows in a large sauce pan, stirring frequently with a spatula.

When marshmallows are completely melted, turn off heat and stir in the cinnamon and vanilla extract.

Add the cereal, and stir until fully combined.

Spread the mixture in a buttered 13x9-inch baking dish, and press down with spatula until even.

Refrigerate until the cereal is firm, and then cut into squares.