Preheat the oven to 425°F (220°C).

Grease 4 ramekins with butter and dust with flour, then place them on a baking sheet.

In a double boiler or microwave-safe bowl, melt 1/2 cup unsalted butter and 1 cup semi-sweet chocolate chips together until smooth, stirring frequently.

Remove the chocolate mixture from heat and whisk in 1 cup powdered sugar until fully incorporated.

Whisk in 2 large eggs, 2 egg yolks, and 1 teaspoon vanilla extract until the mixture is smooth.

Gently fold in 1/2 cup all-purpose flour and 1/4 teaspoon salt until just combined.

Divide the batter evenly among the prepared ramekins, filling each about 3/4 full.

Bake for 12 minutes, until the edges are set but the centers are still soft and jiggly.

Let the cakes sit for 1 minute, then carefully run a knife around the edges and invert onto serving plates.