Prepare the pan by greasing a 9x13 inch baking pan with butter or lining it with parchment paper.

In a large saucepan over low heat, melt 3 tablespoons of unsalted butter, about 2 minutes.

Add 10 ounces of marshmallows and stir constantly until completely melted and smooth, about 3-5 minutes.

Stir in 1/2 teaspoon of vanilla extract if using, mixing until combined.

Remove the saucepan from heat and gently fold in 6 cups of Rice Crispy cereal until evenly coated.

Transfer the mixture into the prepared pan, using a spatula to spread it evenly and pressing down gently.

Allow the treats to set at room temperature for 30 minutes until firm.

Once set, cut into 12 squares and serve.