Preheat the Oven - Set the oven rack to the middle position. Preheat the oven to 450°F (232°C).
Prepare the Ramekins - Melt 1 tablespoon of butter in a microwave-safe dish, for about 30 seconds. Brush it on the inside of four 8-ounce ramekins. Lightly coat with flour and tap out the excess. Set the ramekins on a baking sheet.
Melt the Chocolate - In a double boiler, over simmering water, melt 8 tablespoons of butter with the chocolate. Whisk to combine until smooth, then remove from heat.
Combine the Ingredients - Using a stand mixer with the whisk attachment or hand-held mixer, beat the eggs and egg yolks, sugar, and salt at high speed until thickened and pale, about 3 minutes. Quickly fold the chocolate into the mixture. Fold in the gluten-free flour until just combined.
Add the Nutella - Divide the batter evenly among the prepared ramekins, filling each about halfway. Add 1 tablespoon of Nutella into the center of the batter in each ramekin. Then, evenly distribute the remaining batter on top to cover the Nutella.
Bake - Bake on the baking sheet for 9 minutes, or until the sides are set, the tops slightly cracked, and the centers are slightly jiggly.
To Serve - Let the cakes cool in the ramekins for 1 minute. Run a knife or small spatula along the sides a few times to release.Cover each ramekin with an inverted dessert plate. Wear an oven mitt and carefully turn each one over, and let stand for 10 seconds. Use a spatula or knife to lift the corner of the ramekin and a gloved hand to lift and unmold the cake. Add desired toppings. Serve immediately.
Notes
Do Not Over-Bake: Each additional minute will result in a noticeably less molten chocolate cake filling.
Make Ahead: Portion the batter in ramekins and cover with plastic wrap. Refrigerate for several hours before baking or up to 24 hours in advance. Bake straight from the refrigerator for 10 to 11 minutes.
Butter Substitute: Substitute coconut oil for the butter.
Using All-purpose flour: Substitute all-purpose flour with gluten-free flour.