Grease an 8-by-8-by-2-inch pan. Line with parchment paper, making sure it overlaps the sides so it is easy to take out the fudge once it has set.
Add the butter, brown sugar, milk and marshmallows to a medium pan over low heat to melt gently. Once the sugar has dissolved, turn up the heat and bring to a boil. Boil for 5 to 6 minutes. Take the pan off the heat and add the chocolate. Let it sit for 1 minute, then stir the mixture together until the chocolate has melted. Pour into the prepared pan and let cool for 2 to 3 hours.
When it is set, remove the fudge from the pan, cut into 25 squares and serve.