Create a double boiler by placing a heat-proof bowl over a saucepan filled with a few inches of gently simmering water.

Add the butter, bittersweet chocolate, and semi-sweet chocolate to the bowl.

Stir occasionally until melted and smooth.

Remove from the heat and let the mixture cool for 5 minutes.

After the chocolate mixture has rested, add the confectioners’ sugar, whole eggs, egg yolks, flour, and vanilla extract.

Stir well until everything is fully incorporated.

Generously spray custard cups or ramekins with non-stick cooking spray.

Place them on a baking sheet.

Fill each ramekin about three-quarters full with the batter.

Place the baking sheet with the ramekins in the preheated oven.

Bake for 13 minutes, or until the edges are set but the center still looks slightly soft.