Place butter and chocolate in the top of a double boiler over simmering water. Stir frequently, scraping down the sides with a rubber spatula to avoid scorching, until chocolate is melted, about 5 minutes.

Meanwhile, combine eggs and egg yolk in a bowl. Add sugar and whisk until mixture is light yellow in color and sugar is dissolved, about 3 minutes.

Pour some of the warm chocolate mixture into the egg mixture, whisking quickly to avoid scrambling the eggs. Add remaining chocolate and whisk until combined and smooth. Sift in flour, cocoa powder, and salt; fold in with a spatula until combined.

Butter four 5-ounce ramekins and dust with cocoa powder. Spoon batter into the prepared ramekins, tapping gently to settle any air bubbles. Place in the refrigerator for 30 minutes.

Preheat the oven to 425 degrees F (220 degrees C).

Place ramekins in a baking dish and bake until edges start to pull away from the sides, 13 to 15 minutes.

Remove from the oven and dust with powdered sugar. Serve warm.