Peanut Butter–Chocolate Fudge

1 (14-ounce) can sweetened condensed milk

1 (11.5-ounce) package bittersweet chocolate chips

1 1/2 cups miniature marshmallows

1 cup smooth peanut butter

1 teaspoon vanilla extract

Coat an 8-by-8-inch baking dish with butter and set aside.

Combine milk, chocolate chips, and marshmallows in a medium saucepan over medium heat. Cook, stirring constantly, until mixture is melted and smooth, about 5 minutes.

Remove from heat and mix in peanut butter and vanilla extract until thoroughly combined.

Turn into the prepared baking dish, spread out evenly, and let cool to room temperature, about 10 minutes.

Cover and place in the refrigerator until firm, about 1 hour. Cut into 25 squares and serve.