Position oven racks so the cakes will bake in the middle of the oven; preheat oven to 400F.
Grease and line six 6-ounce custard cups or souffle dishes.
Here's how; using a pastry brush, lightly grease the bottom and sides of each pan with room-temperature solid shortening; place each pan on a sheet of parchment paper and trace around the bottom of the pan with a pencil, then cut out the traced shape with scissors; lay the parchment in the pan and use your hand to smooth out any wrinkles; if desired, lightly brush the paper with more solid shortening for easy removal of the paper after baking.
Melt the chocolate and set aside to cool slightly.
In a bowl or food processor, combine the butter, granulated sugar, salt and eggs and beat with an electric mixer at medium speed or process until well blended; add the flour and blend well.
Add the vanilla and cooled chocolate and blend until smooth.
Divide the batter evenly among the prepared baking dishes and smooth the surfaces with a small rubber spatula.
Place on a baking sheet, transfer to the oven, and bake until the tops are well puffed, about 17 minutes.
Remove the baking dishes to a wire rack to cool for about 5 minutes, then run a thin knife blade around the sides of each cake and invert directly onto individual serving plates; peel off the parchment.
Sift a little powdered sugar over the tops of the cakes.
James McNair recommends serving by spooning custard sauce or creme anglaise around each cake, then add dollops of mango and raspberry purees and swirling with a wooden skewer; he also recommends using edible gold leaf as the garnish instead of powdered sugar.