Step 1 In a bowl set over a pot of simmering water, melt the unsweetened chocolate and butter. Remove from the heat and cool slightly.
Step 2 In the bowl of a mixer with a paddle attachment, or in a large bowl using a fork, combine the eggs, vanilla and sugar. Mix just until incorporated and set aside.
Step 3Sift together the flour, cocoa powder, baking powder and salt into a medium bowl. Set aside.
Step 4Add the melted chocolate to the egg mixture and mix just until combined. Stir in the sifted dry ingredients and mix just until combined, then stir in the bittersweet chocolate.
Step 5Cover the batter with plastic wrap and refrigerate for at least 1 hour to chill thoroughly. Heat the oven to 350 degrees.
Step 6Divide the dough into 18 portions. Grease your hands (to prevent the dough from sticking) and shape the portions into balls. Place the balls on a greased, parchment-lined sheet pan, leaving 2 to 3 inches between each.
Step 7Bake until the edges of the cookies are just set and the center is still soft, 10 to 12 minutes, rotating the pan halfway through. Place the cookies, still on the parchment, on a rack and cool completely before serving. They will be very soft.