Preheat the oven to 425 degrees F. Spray a six-count muffin tin or four 6-oz ramekins with non-stick cooking spray. Dust the inside of each cup thoroughly with cocoa powder, tapping out any excess. Set aside.

Place the chocolate chips and butter in a microwave-safe dish. Heat in 30-second increments, stirring well between each interval, until completely melted and smooth.

Allow the chocolate mixture to cool to room temperature before proceeding.

In a small bowl, combine the flour, powdered sugar, and salt. Whisk to combine. Set aside.

In another small bowl, add the eggs and egg yolks. Whisk to combine. Set aside.

Add the dry ingredients and the egg mixture to the cooled melted chocolate. Whisk to combine well until the batter is glossy and smooth.

Pour the batter evenly into the prepared baking dishes. Place ramekins on a baking sheet or place muffin tin directly on the middle rack.

Bake in the preheated oven for approximately 14 minutes. The tops should be set and beginning to crack. Remove from the oven and allow to cool for approximately 5 minutes.

Using a butter knife, gently run it along the edges of each cake to release anywhere that might have stuck. Place a plate or serving dish on top of the cakes and quickly invert. Use caution and oven mitts - the pans will still be very hot.

Allow the cakes to cool slightly. Dust with additional cocoa powder or powdered sugar. Serve immediately with ice cream, hot fudge, berries, or your favorite toppings.