Line a 9x13-inch pan with parchment; grease lightly with the extra butter.
Warm butter in a large pot over medium-low heat.
Stir in marshmallows; cook, stirring, until fully melted and smooth.
Remove from heat. Mix in vanilla and a small pinch of salt.
Add cereal and fold until every piece is coated.
Press mixture into the pan with a buttered spatula, gently.
Let cool until set, then cut into squares.