Preheat oven to 425 degrees. Spray four 6 oz ramekins with cooking spray and lightly dust with cocoa powder.
Place butter and chocolate in a large microwave safe bowl and microwave in 15 second increments, stirring each time until completely smooth. Set aside.
In a medium-sized bowl, whisk together the flour, sugar, and salt. In a small bowl whisk together the eggs and egg yolks.
Pour both the flour mixture and eggs into the bowl with the chocolate and gently stir until combined and smooth. Divide the batter between the ramekins, place on a baking sheet, and bake for 10-13 minutes. Let cool for one minute, then serve with a dusting of powdered sugar.