Line slow cooker with parchment paper and lightly spray with cooking spray.
In a mixing bowl, combine graham cracker crumbs, ¾ cup melted butter and ½ cup sugar. Press mixture into the bottom of lined slow cooker.
In a medium saucepan over medium heat, melt ½ cup butter and the bitter-sweet chocolate chips together; remove from heat, and mix in ¾ cup sugar and eggs, one at a time. Stir in flour, cocoa powder, baking powder, salt and semi-sweet chocolate chips.
Spread batter over graham cracker crust. Place Hershey bars evenly on top of the brownie batter. Cover and cook on low 2 - 2 ½ hours.
When brownies pass the toothpick test (check at 2 hours), top with marshmallows and replace lid for 5-10 minutes until marshmallows are melted.
If desired, place slow cooker crock under broiler for a minute or use a kitchen torch to toast the marshmallows.
Use parchment to lift brownies from dish to serve.