Combine 6 oz of 70% chocolate, 4 oz of dark chocolate and the 1 stick of butter in a microwave-safe bowl. Heat on high for 30 seconds. Mash mixture around and continue heating on high and stirring every 30 seconds until melted and smooth. Set aside to cool.
Mix the flour, baking powder, and salt in a bowl and set aside until needed.
Put the eggs, sugar, coffee and vanilla in the bowl of a mixer fitted with the whisk attachment and beat at high speed for about 10 minutes or until the mixture is very thick and yellow.
Very gradually add the (just slightly warm) chocolate. Scrape down the sides of the bowl and work rubber spatula around the bottom of the bowl, then continue to mix just until the chocolate is incorporated. Add the dry ingredients and mix gently but thoroughly. Make sure the mixture is not so warm that it will melt any additional chocolate, then stir in the remaining chocolate chunks.
Cover the bowl with plastic and chill for several hours, or overnight. The dough can be made ahead and kept refrigerated for up to 4 days.
Baking the cookies: When you are ready to bake, position the racks to divide the oven into thirds and preheat the oven to 350˚F.
Line two large baking sheets with parchment paper.
Using a heaping tablespoon of dough for each cookie, drop the dough onto the lined sheets, leaving at least 2 inches of space between each mound of dough.
Bake the cookies for 10 to 12 minutes, rotating the pans front to back and top to bottom halfway through the baking period. The cookies will puff, then sink and crinkle and wrinkle around the edges. Note: If you are baking with dough that is room temperature rather than super cold, check the cookies at 8 minutes. These are much better when NOT over-baked.