Preheat your oven to 425°F (220°C). Grease four ramekins with butter and dust them with cocoa powder, tapping out the excess. Place them on a baking sheet.

In a microwave-safe bowl, combine the unsalted butter and semi-sweet chocolate chips. Microwave in 30-second intervals, stirring in between, until the mixture is completely melted and smooth. Allow it to cool slightly.

In a mixing bowl, whisk together the eggs, egg yolks, and granulated sugar until the mixture is pale and slightly thickened.

Stir in the vanilla extract into the egg mixture.

Gradually pour the melted chocolate mixture into the egg mixture, stirring constantly until well combined.

Sift the all-purpose flour and salt over the chocolate mixture. Gently fold the dry ingredients into the wet ingredients until just combined; be careful not to overmix.

Evenly distribute the batter into the prepared ramekins, filling each about 3/4 full.

Bake in the preheated oven for 12-14 minutes, or until the edges are firm but the center is soft. The tops should look set.

Remove from the oven and let the cakes rest for 1 minute. Carefully invert each ramekin onto a plate and gently lift them off.

Serve immediately while the centers are still gooey. Enjoy with a dusting of powdered sugar, fresh berries, or a scoop of vanilla ice cream.