Melt the butter over low-medium heat in a large pot. Allow it to get brown and then add in the corn syrup. Bring to a simmer and allow to bubble for about 2 minutes.
Remove from the heat and stir in the marshmallow fluff, the vanilla and the mini marshmallows (if using). Stir until well combined.
Stir in the Rice Krispies and stir until all well combined.
Press into an 8x8 pan, allow to cool and then cut into squares and serve.
Keyword Rice Krispie Treats with Marshmallow Fluff