Heat oven to 450 F.  Brush a 6-ounce ramekin or baking cup with melted butter, making sure not to miss any spots (I usually just melt a little more than the 27g total and then use some of it to coat the ramekin, and the rest in the cake). Spoon a little cocoa powder in and pat it around so it coats the bottom and sides, then knock the excess out.  

In a medium heatproof bowl, put the butter and chocolate pieces and melt them about two-thirds of the way.  There are two ways you can do this, either in the microwave or set over a pot of simmering water---I do the microwave, about 15-20 seconds at a time and stirring in-between.  You don't want it all the way melted because it will be too hot to add the egg.

Whisk until it finishes melting and then whisk in the sugar and salt.  Whisk in the egg and beat it until mixed in, and THEN add a good 20 to 30 extra times.  This ensures that the mixture is smooth, glossy, and lightened a bit.  DO NOT SKIP THIS.  It makes a huge difference in your final cake texture.  See pics above for what it looks like.

Finally, add the cocoa powder and whisk until combined.

Spoon the batter into your prepared ramekin.  If you're adding a filling, see notes below on how to do this (peanut butter is a fave of mine!). 

Bake 7 to 9 minutes (7 will be the most molten in the center, 9 still plenty gooey; mine always need 8 or 9 minutes).  Transfer to a cooling rack for a minute.  I take my mini offset spatula or a butter knife and very lightly go around the edge of the cake, just to loosen it more cleanly.

Then (wearing pot holders), place a small dessert plate over the ramekin and carefully flip the whole thing over.  Count to ten, maybe give it a gentle shake, then pull the ramekin away...leaving (hopefully) your molten lava cake.

Dust with powdered sugar and serve as-is or with berries, barely sweetened whipped cream, or a little ice cream. Eat immediately.