In a medium saucepan over medium heat, combine semi-sweet chocolate chips, sweetened condensed milk, and miniature marshmallows. Stir frequently for about 5 minutes until the mixture is melted and smooth, ensuring no lumps remain.

Once your chocolate mixture is silky smooth, remove the saucepan from heat. Quickly stir in the creamy peanut butter and vanilla extract until fully combined and creamy.

Lightly grease a 10-inch square pan with butter or line it with parchment paper for easy removal later.

Pour the delightful chocolate-peanut butter mixture into the prepared pan. Using a spatula, spread the mixture evenly to fill the pan completely.

Place the pan in the refrigerator and chill for about 3 hours, or until the fudge is firm to the touch.

Once chilled and firm, remove the fudge from the refrigerator. Carefully lift it out of the pan using the parchment paper, if used, and slice it into 1-inch squares.