Grease a 9" x 13" baking dish with butter or cooking spray. Set it aside.
In a large saucepan, melt the butter over low heat. Once melted, add the marshmallows to the saucepan. Stir the marshmallows continuously using a rubber spatula until they completely melt. This usually takes about 10 minutes.
8 tablespoons (1 stick) unsalted butter, 20 ounces (2 packages) marshmallows
Stir the vanilla extract and kosher salt into the melted marshmallows.
1 teaspoon vanilla extract, ¼ teaspoon kosher salt
Remove the saucepan from the heat. Add the crispy rice cereal to the marshmallow mixture. Stir everything together until the cereal is evenly coated with the marshmallow sauce.
10 cups crispy rice cereal
Quickly stir in half of the mini chocolate chips. They may melt a bit due to the warm marshmallow mixture, but that's okay.
1 cup mini chocolate chips
Dump the mixture into the greased baking dish and spread it out evenly. Use a piece of greased parchment or wax paper to press down on the crispy rice mixture gently.
Sprinkle the remaining mini chocolate chips over the top of the rice krispie treats. Press them in lightly so they stick to the surface.
1 cup mini chocolate chips
Allow the treats to cool at room temperature for at least 30 minutes. Once cooled and set, slice them into squares or rectangles for serving.