Preheat oven to 425°F. Grease a 6-inch cake pan, line it with parchment paper, and spray more cooking spray on top of the parchment paper.
In a large microwavable bowl, place the chopped chocolate and butter then microwave for 1 minute. Stir the chocolate and butter together until melted.
In a medium bowl, beat together the eggs, egg yolks, and sugar until pale and thick in texture.
Gradually beat the egg mixture into the melted chocolate-butter until completely incorporated.
Add the flour and salt to the egg and chocolate mixture and beat until just combined.
Pour the batter into the prepared cake pan and bake for 12 to 14 minutes.
The cakes should be slightly under-cooked, with the center still gooey.