Preheat the oven to 425 F.

Spray four 6-ounce ramekins with nonstick cooking spray or lightly grease with shortening. Dust lightly with cocoa powder.

Place a large stainless steel bowl or the top of a double boiler over simmering water. Add the butter and chocolate and cook until melted, stirring constantly. Remove the bowl from the heat.

Add the confectioners' sugar to the chocolate mixture and blend well.

In another bowl, whisk the eggs; add the eggs to the chocolate mixture, blending thoroughly. Add the vanilla and then stir the flour and salt into the mixture.

Place the ramekins on a baking sheet. Fill the ramekins with the chocolate mixture.

Bake the cakes just until firm, but still soft in the center, about 13 to 14 minutes.

Remove the cakes from the oven and let them rest for 2 minutes. Invert onto dessert plates.

Sift powdered sugar over the cakes, if desired.