Preheat the oven to 450℉.
Grease 6 ounce ramekins (3.5-inch in diameter) with melted butter, making sure to get every nook and cranny, and then apply a light dusting of cocoa powder.
Extra melted butter for greasing ramekins, Cocoa powder or flour for dusting ramekins
Add the chocolate and butter to a bowl and melt in the microwave using 30 second increments, stirring between each time. If you prefer, you can also use the double boiler method by placing the bowl of chocolate and butter on top of a pot of simmering water, stirring constantly until the chocolate is smooth.
6 ounces semi-sweet chocolate baking bar (I used Ghirardelli 60%), broken into small pieces, ½ cup unsalted butter
In another bowl, whisk together the eggs, egg yolks, and vanilla extract.
2 large eggs, at room temperature, 2 large egg yolks, at room temperature, ¼ teaspoon vanilla extract
Separately, stir together the flour, powdered sugar, and salt.
2 Tablespoons all purpose flour, ¼ cup powdered sugar, Pinch of salt
Now add the egg mixture and the flour mixture to the melted chocolate, and gently fold all the ingredients together. The cake batter will be thick and a little lumpy and that's okay, just try to blend in any large clumps of flour as best as possible.
Divide the batter evenly between the four prepared ramekins. Each ramekin will be about two-thirds full with the cake batter.
Place the filled ramekins on a baking sheet and bake on the middle rack of the oven for about 9 to 12 minutes.
Now here's where it gets a little bit tricky. You want the cake edges to be set firm, but the inside of the cake still jiggly with hot lava. Start checking the cakes at about the 9 to 10-minute mark. Give the pan a gentle shake and take a look at the cakes. Does it seem like the sides are set, but they're not really completely baked through? Perfect!
Remove from the oven and let them cool for just a minute or so.
Using two pot holders, pick up the ramekin from the top with one hand and place it in the other hand so you're holding it. Then, place an inverted dessert plate on top and flip it over.
Place the plate on the countertop and give the bottom of the ramekin a quick tap or two with your hand. After about 10 seconds, gently lift the ramekin up to reveal the best chocolate molten lava cake.
Serve immediately and enjoy it on its own or with a variety of fun toppings, like ice cream, whipped cream, powdered sugar, and berries!