Add butter and chocolate to a large, microwave safe bowl. Heat for 30 second intervals, stirring well between each time. (1-1 1/2 minutes will be enough.) Stir until completely smooth.
Add powdered sugar and stir until smooth again.
Add eggs, egg yolk, vanilla and salt, stirring to combine.
Stir in flour and graham cracker crumbs.
Spray (4) 6 ounce ramekins with cooking spray. Fill 1/3 of the way and then add a scoop (about 1 tbsp) of marshmallow creme in the center of the batter. Top with additional batter until each ramekin is full to the line. Don't overfill, as they do puff up a bit when cooking. There may be a bit of batter left in the bowl, depending on how much marshmallow creme you use.
Place trivet in the pot and add 1 cup of water. Set 3 ramekins down on the trivet and then set the last one on top.
Close and lock the lid, turning the steam release valve to closed. Press Steam and adjust time down to 8 minutes. Press Start/Stop.
When cooking cycle is done, do a quick release and carefully remove the ramekins. (I used tongs for this.) Let them cool for 1-2 minutes.
Invert the ramekins onto a plate and let the cake slide out. Top with additional marshmallow creme and graham cracker crumbs. Serve immediately!
Notes
For a regular chocolate lava cake, replace the 3 tbsp of graham crackers with 3 tbsp of flour and eliminate the marshmallow creme. Cook as directed and serve topped with powdered sugar or ice cream! This recipe was tested in a 6 quart Crockpot Express using standard porcelain 6 ounce ramekins. For an 8 quart cooker, use 1 1/2 cups of water and the 4 ramekins should nestle right next to each other inside the pot. Cook time may be a bit less. Start with 7 minutes and adjust as needed.