Step 1In a medium bowl, sift together the flour, cocoa powder and salt.
Step 2In the bowl of a mixer fitted with the paddle attachment, mix together the eggs and sugar on medium speed until thick and pale in color, 3 to 5 minutes. Add the vanilla and mix until incorporated.
Step 3With the mixer on low speed, add about one-third of the dry ingredients, then one-third of the butter, and continue alternating with the remaining flour and butter. Add the chocolate and continue to mix to combine. (The batter can be made up to this point and refrigerated, covered, for up to one day.)
Step 4Heat the oven to 350 degrees. Butter and flour 12 timbale molds. Set aside.
Step 5Put 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 two-thirds full. Bake the bouchons until the tops are shiny and set (like a brownie), and a toothpick inserted in the center comes out moist but clean (there may be some melted chocolate from the chopped chocolate), 20 to 25 minutes.
Step 6Transfer the bouchons to a cooling rack. After a couple of minutes, invert the molds and let the bouchons cool in the molds. Remove the molds and serve, or store until needed (the bouchons are best eaten the day they are baked).
Step 7To serve, invert the bouchons and dust them with powdered sugar. Serve with ice cream, if desired.