In a bowl set over simmering water, melt the unsweetened chocolate and butter. Remove from the heat and cool slightly.
In a bowl of a mixer with a paddle attachment, or in a large bowl using a fork, combine eggs vanilla, and sugar. Mix just until incorporated and set aside.
Sift together the flour, cocoa powder, baking powder and salt into a bowl and set aside.
Add the melted chocolate to the egg mixture and mix just until combined. Stir in the sifted dry ingredients mix just until combined. Then stir in the bittersweet chocolate.
Cover the batter with plastic wrap and refrigerate for at least 1 hour to chill thoroughly. Heat the oven to 350°F.
Divide 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-line sheet pan, leaving 2 to 3 inches between each.
Bake until the edges of the the cookies are just set and the center is still soft, 10 to 12 minutes- rotating the pan halfway through. Place the cookies with the parchment on a rack to cool completely before serving.