The next step is to melt the chocolate and butter together. You can do this in the microwave or on the stovetop. Here's how:

In a microwave-safe bowl, combine the butter and chocolate chips.

Microwave the mixture in 30-second increments, stirring after each interval, until the butter and chocolate are completely melted and combined.

If you prefer to melt the chocolate and butter on the stovetop, you can do so by using a double boiler. Simply fill a saucepan with an inch of water and place a heatproof bowl on top, making sure the bowl does not touch the water. Bring the water to a simmer, then add the butter and chocolate chips to the bowl. Stir until the mixture is melted and smooth.

In a separate mixing bowl, whisk together the granulated sugar, eggs, and egg yolks until well combined.

Slowly pour the melted chocolate and butter mixture into the egg mixture, whisking continuously until the two mixtures are completely combined.

Next, add the all-purpose flour and salt to the chocolate and egg mixture, and stir until the dry ingredients are fully incorporated.

Preheat your oven to 425°F. Grease four 6-ounce ramekins with cooking spray or butter. Divide the batter evenly between the ramekins, filling each one about three-quarters of the way full.

Place the ramekins on a baking sheet and bake for 12-14 minutes, or until the edges of the cakes are set but the centers are still slightly jiggly.