Line an 8x8 baking pan with aluminum foil or parchment paper, making sure to leave some overhang for easy removal and set aside.

Heat a medium saucepan over low-medium heat (you may also use a double boiler for this step as well). Add the sweetened condensed milk, chocolate chips, and butter and stir well until the mixture is completely melted and smooth. Remove from the heat and set aside for a few minutes.

Add the graham cracker pieces to the mixture and stir well to combine. Gently fold in the mini marshmallows until just combined, making sure not to over mix.

Scoop the mixture into the prepared pan and gently smooth it out. If you prefer large pieces of marshmallows in the fudge, scoop half of the mixture into the prepared pan and smooth it out, sprinkle a small handful of mini marshmallows on top, add the rest of the fudge on top, and smooth it out.

Transfer to the refrigerator and refrigerate for at least 3 hours or until the fudge is solid enough to cut.

Remove from the refrigerator and cut into small pieces.