1. Heat oven to 325 degrees . In a small bowl, stir together graham cracker crumbs, melted butter, sugar and salt. Spoon 3 tbsp crumb mixture into each jar. Press mixture with a small spice jar to flatten into bottom.
2. Spoon 1/4 cup marshmallow creme into each jar and set jars aside. (Marshmallow creme should spread to edge of jar; spread level with the back of a greased spoon, if necessary.) Meanwhile, prepare brownie mix as per package instructions, using egg, oil and 1/4 cup water. Divide batter among jars, using a heaping 1/4 cup per jar. Bake at 325 degrees for 30 minutes. (Marshmallow may puff up in center of jar.)
3. Heat peanut butter in microwave for 30 seconds. Spoon 1/2 tbsp peanut butter into each jar. Top with a little of the remaining graham cracker mixture and 1/3 cup miniature marshmallows. Increase oven heat to broil. Broil s'mores 4 inches from heat for 30 seconds to 1 minute, until lightly browned. Cool slightly before serving.