Yield: makes 2 dozen
Preheat oven to 350 degrees F.
Over a bain-marie (water bath), heat 8 oz chocolate chips and butter until almost melted. Do not overheat. Chocolate butter mixture should be shiny and still in temper.
In another bowl, whisk together flour, baking powder, salt, and espresso powder. Set aside.
In the bowl of a stand mixer, beat together eggs, brown sugar, and vanilla until fluffy and light in color. Add melted chocolate and mix on low speed. Add dry mixture and mix until just combined. Fold in remaining 12 oz chocolate chips.
Scoop tablespoons of dough 2 inches apart onto a parchment lined baking sheet. Bake for 10-14 minutes until cookies are shiny, crackly and still soft in the center. Cool on baking sheet for 10-15 minutes before letting cool completely on a wire rack.
The batter will look more like a brownie batter than a cookie dough. If you overheat your chocolate butter mixture, your cookies will not be as shiny.
ADAPTED FROM MARTHA STEWART