Preheat the oven to 425°F. Spray 6 (6-ounce.) ramekins with baker's spray and lightly dust them with the cocoa powder. This helps to ensure that the chocolate lava cakes come out cleanly from the ramekins. Place all the prepared ramekins onto a small rimmed baking tray. Set aside.
In a medium bowl, melt the broken-up pieces of semi-sweet chocolate bars and the cubed butter in the microwave for 1 minute. Stir gently and heat in 15-second intervals if needed. Stir to combine the melted chocolate and butter completely.
Stir in the vanilla extract, then the powdered sugar, and lastly, the eggs until well combined. Make sure to combine completely before adding the next ingredient.
Add the salt and all-purpose flour. Stir just until fully incorporated. Do not over mix the batter.
Using a large 3-inch ice cream scoop, scoop the batter evenly into each prepared ramekin about ⅔ full. Each scoop will be approximately ½ cup batter and will perfectly divide your batter into 6 portions.
Bake the chocolate lava cakes for 12 to 14 minutes. Be very careful not to overbake your lava cakes, or you will lose the gooey center. They should just barely look done on the top.
Once your chocolate lava cakes have come out of the oven, let them cool on the tray for 2 minutes before inverting them onto a serving plate. Be very careful because the ramekins will be very hot.