In a small bowl, toss 1/2 cup of chocolate chips with 1 TBSP brownie mix. (This will help keep the chips from all sinking in the mix).
Combine brownie mix, egg, oil, and water in a medium mixing bowl.
Mix on low to medium speed until combined.
Fold in marshmallows and dusted chocolate chips.
Pour into crust and bake for 30-35 minutes (crust will be quite full, a cookie sheet underneath is a good idea) or until it is set - gooey is good, liquid is bad.
Let pie cool completely.
While the pie is baking, heat cream in a small saucepan. until the first bubbles appear. Do not bring to a full boil.
Pour hot cream over the mini chocolate chips (mini chips will melt better than standard size, but you can use any finely chopped chocolate).
Let the cream and chips stand for a minute to soften, then stir until smooth.
The ganache needs to be at room temperature to whip. You can either chill it completely and then let it sit on the counter at room temperature for a half hour, or just pop it in the freezer for 15-30 minutes then let it sit at room temperature. Once you have slightly cool chocolate ganache, whip it on high until thick and fluffy, like mousse.
Top pie with whipped ganache.
You can decorate with mini marshmallows and/or chocolate candies (Hershey Kisses are lovely for this) or eat as is.