Thoroughly grease four 8-ounce ramekins with unsalted butter. Proceed by dusting the insides of the ramekins with all-purpose flour, ensuring the removal of any excess flour.

Position a mixing bowl above a medium saucepan filled with approximately 1 cup of water, ensuring that the bottom of the bowl does not make contact with the water. Place the 6 ounces of semisweet chocolate chips and sliced butter into the bowl. Maintain the water at a gentle simmer and stir the chocolate and butter consistently until both are melted and amalgamated. Once combined, remove the mixture from heat and set it aside to cool slightly.

In a separate mixing bowl, amalgamate the two whole eggs, two egg yolks, 1/4 cup granulated sugar, and 1/4 teaspoon salt. Using an electric mixer at high speed, beat the mixture for approximately 5 minutes, or until the result is a thick, light yellow consistency.

Gradually introduce the slightly cooled chocolate mixture into the egg mixture, followed by the addition of the pure vanilla extract. Gently fold to combine. Subsequently, introduce the 3 Tablespoons of all-purpose flour and fold until the mixture is homogenized.

Distribute the batter evenly among the prepared ramekins. Place the ramekins in a preheated oven at 450°F (232°C) and bake for 10-12 minutes. The cakes should display a firm and dry exterior, with a soft center that exhibits minimal movement.

Upon removal from the oven, allow the cakes to cool for 1 minute. If necessary, use a small knife to gently loosen the edges of the cakes. Promptly invert each cake onto serving plates. Dust with powdered sugar if desired, and serve while warm.