Grease 9 x 13 with extra butter and set aside
Melt 4 Tablespoons butter in large pot over medium-low heat.
Once melted, add 4 cups mini marshmallows
Stir until marshmallows are completely melted
Remove from heat
Stir in vanilla
Add cereal and stir until combined
Add in the rest of the marshmallows and stir until the marshmallows just start to melt
Press into buttered 9 x 13 pan
Allow to cool a few minutes before cutting