While the crust cooks, melt the butter in a small bowl, in short intervals, as described for the crust. Allow to cool slightly before proceeding ~2 minutes.
Mix brown sugar, vanilla, an egg, and salt into the butter. Make sure all ingredients are well combined before proceeding.
Add in flour and mix until just combined.
Gently mix in chocolate chips (or your chosen combination of chips) until just combined.
Pour your mixture over the cooked crust. Use a spatula to spread the mixture into an even layer. The layer may seem thin but it will do its job.
Cook the bars for another 10 minutes.
Pour the marshmallows over the top of the bars. You want an even layer that is about 1 - 1 ½ inch thick. That generally takes about a half a bag of mini marshmallows for me. Don't skimp on this step.
Cook the bars for 15 minutes, until the marshmallows are toasty brown.
Remove bars from the oven and let cool, if you can, until marshmallows have set, before cutting them.