In a saucepan on medium, combine butter and brown sugar. Stir continuously. When it comes to a boil, slowly pour mixture over crackers and spread with a spatula. It doesn't have to be perfect!
Bake 3 minutes.
Remove from oven and sprinkle with chocolate chips, allow a minute or two to melt, smoothing the chocolate with a spatula.
Top with marshmallows and return to the oven for 2 minutes, or until slightly toasted.
Allow the toffee to cool and set. You can refrigerate to speed the process and then cut into pieces.
Notes
The options are endless! Customize with peanut butter cups and more.