Preheat your oven to 425°F. Spray a six-count muffin tin or four to six individual ramekins with non-stick cooking spray, then dust them with cocoa powder. Place the chocolate chips and butter together in a microwave-safe bowl. Microwave in 30-second bursts, stirring well between each interval, until the mixture is completely melted and smooth. Set the bowl aside and allow it to cool to room temperature — this step is important because adding hot chocolate to the eggs will scramble them.

While the chocolate cools, whisk together the flour, powdered sugar, and salt in a small bowl. In a separate bowl, whisk together the whole eggs and egg yolks until combined. Once the chocolate mixture has cooled slightly, add both the dry ingredients and the egg mixture to it. Whisk everything together until the batter is smooth and well combined.

Pour the batter evenly into your prepared baking dishes, filling each about three-quarters of the way. If using ramekins, place them on a baking sheet. If using a muffin tin, place it directly on the middle oven rack. Bake for approximately 14 minutes, until the tops are set and just beginning to crack.

Remove the cakes from the oven and let them cool for about 5 minutes. Run a butter knife gently around the edges of each cake to release any spots that may have stuck.

Place a plate or serving dish on top of the pan, then carefully and quickly flip it over to invert the cakes. Use oven mitts — the pans will still be very hot! Dust with a little cocoa powder or powdered sugar and serve immediately.