I found this recipe in an old copy of Real Simple magazine. I looked it up online to share the recipe directly from them and then will post it here as well.
What you’ll need
8 tablespoons (1 stick) unsalted butter, plus extra for coating
3 tablespoons all-purpose flour, plus extra for dusting
1/2 cup sugar, plus extra for dusting
10 ounces semisweet chocolate, coarsely chopped
4 large eggs
1 large egg yolk
1 teaspoon vanilla extract
1/4 teaspoon salt
1/2 cup mini marshmallows
2 tablespoons unsweetened cocoa powder
What to do
Preheat oven to 375° F. Generously butter, flour, and sugar eight 6-ounce coffee cups or mugs made of ovenproof stoneware (see tip below). Wipe the rims clean.
Place the 8 tablespoons of butter and the chocolate in a medium heatproof bowl over a pan of simmering water and melt, stirring once or twice until smooth. Remove from heat and let cool 5 minutes.
Using a handheld mixer, beat the eggs, yolk, vanilla, salt, and 1/2 cup sugar at the highest speed until the volume doubles and the mixture becomes foamy, about 5 minutes. Stir the 3 tablespoons of flour into the chocolate, then gradually add it to the egg mixture, beating on low until it’s fully incorporated. Ladle the batter into each cup until it’s about 1/2 inch from the rim.
Bake until the cakes puff and begin to crack but the centers are still a bit runny, 13 to 17 minutes. Remove from oven. Sprinkle with marshmallows, then return to oven for 2 to 4 minutes, until the marshmallow tops begin to crisp. Remove from oven; let cool about 5 minutes. Sift the cocoa on top.
Tip: You can bake these individual cakes in coffee mugs or teacups, or use eight 6-ounce ramekins. (To determine if your mugs are ovenproof, refer to the packaging or check with the manufacturer.) Alternatively, a single cake can be made in a 10-inch springform pan or pie plate with enough regular-size marshmallows to cover. To do this, increase the initial baking time to 22 to 25 minutes or, if you prefer a more gooey center, to 17 to 20 minutes.
Here is how mine turned out. Not as pretty as their photo but still looks and tasted sooooo gooey yummy delicious. It was really fun to do with my daughter, who loves anything involving chocolate and marshmallows.