Preheat oven to 450 degrees. Prepare ramekins or small bowls (should hold about 4 ounces of cake batter) with baking spray or pan release.
Place chocolate and butter into a microwave safe dish and heat for 30 seconds. Remove and stir well, then heat for 10 seconds more. Stir until no lumps remain. Set aside.
Beat egg yolks, eggs, and sugar in a stand mixer with a whisk attachment on high speed (can also use a hand held mixer) until eggs triple in size, about 2 minutes.
With mixer on medium-low speed, slowly pour the chocolate mixture into egg mixture. Mix until light and thick, about 1 minute.
Remove bowl from stand mixer and fold in flour by hand.
Divide batter between four ramekins. Place ramekins on a cookie sheet.
Bake for 7-8 minutes or until the center does not jiggle. Immediately invert each ramekin on a plate and let sit for at least 10 seconds. Gently remove the ramekin by carefully lifting it up and letting the cake fall onto the plate.