In a medium saucepan over medium-heat, melt the butter. Lower the heat to low, add the large marshmallows, and stir until completely melted.
Remove the saucepan from heat. Stir in the vanilla extract and chocolate, stirring until melted. Add the cereal; stir until evenly coated. Let cool for a couple minutes, then stir in the tiny marshmallows and stir to evenly distribute.
Scoop the mixture into the prepared pan and press to spread it out to fill the pan and gently compact the treats. Cut into 15 bars. Store at room temperature in an airtight container for up to 2 days.