Preheat oven to 325 degrees and place half pint mason jars on baking sheet.
For the crust:
Crush Biscoff cookies in food processor into small crumbs and combine with melted butter.
Press 2 tablespoons of crumb mixture into each jar.
Bake jars for 5 minutes then allow to cool completely.
For the filling:
Heat milk and cream over medium to low heat until it begins to simmer.
In the meantime begin whisking eggs in a small bowl, set aside.
Remove milk and cream from heat, add chocolate chips, then wait for chocolate chips to slightly melt (about one minute). Stir until chocolate is melted and smooth.
Add sugar and a pinch of salt then whisk chocolate.
While whisking the chocolate slowly add eggs, continue whisking to avoid having the chocolate cook the eggs.
Pour chocolate filling into jars, bake for 20 minutes.
Top with miniature marshmallows and warm under the broiler for 1-3 minutes. You want the marshmallows to slightly brown, but watch them carefully after one minute.
Allow s'mores pies to cool, use caution as the jars will be hot.