Sift the flour, cocoa powder, and salt into a bowl; set aside. In the bowl of a mixer fitted with the paddle attachment, or in another large bowl if using a handheld mixer, mix together the eggs and sugar on medium speed for about 3 minutes, or until very pale in color. Mix in the vanilla. On low speed, add about 1/3 of the dry ingredients, then 1/3 of the butter, and continue alternating with the remaining flour and butter. Add the chocolate and mix to combine. (The batter can be refrigerated for up to a day.)

Put the timbale molds on a baking sheet. Place the batter in a pastry bag without a tip, or with a large plain tip, and fill each mold about 2/3 full. (I just scooped the batter in rather than piping it.) Place in the oven and bake for 20 to 25 minutes. When the tops look shiny and set (like a brownie), test one cake with a wooden skewer or toothpick. It should come out clean but not dry (there may be some melted chocolate from the chopped chocolate). Transfer the bouchons to a cooling rack. After a couple of minutes, invert the timbale molds and let the bouchons cool upside down in the molds; then lift off the molds. (The bouchons are best eaten the day they are baked.)

Dust the tops with confectioners' sugar before serving.