Heat oven to 450 F.

Brush two 6-ounce ramekins or baking cups with melted butter, making sure not to miss any spots. Spoon a little cocoa powder in each and pat it around so it coats the bottom and sides, then knock the excess out.  I tried cutting corners here and had them stick every time, so don't skip this (and yes, butter seems to be much better than cooking spray).

In a medium heatproof bowl, put 4 tablespoons of butter and the 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.

Divide between the two ramekins.  If you're adding a filling, see notes below on how to do this. 

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 1 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 your first ramekin. Flip it onto the plate, count to ten, then lift the cup off. Repeat with second cake. Dust with powdered sugar and serve as-is or with berries, barely sweetened whipped cream, or a little ice cream. Eat immediately.