In a small bowl, melt butter and 2/3 cup of the chocolate chips together in the microwave. Heat for 30 seconds and stir. Then continue in 15-second bursts, stirring in between until smooth. Set aside to cool for 5 minutes.
In a medium bowl, whisk together brown sugar, egg, and vanilla until just combined. Whisk in the cooled chocolate mixture.
Mix in flour, cocoa powder, baking powder, and salt, until just combined. Dough will be very soft and sticky.
Cover and chill dough for 45 minutes until the dough is firm enough to handle.*
Preheat oven to 325°Line baking sheet with parchment paper or silicone baking mat.
Scoop and roll dough into 6 large balls and place on prepared cookie sheet spaced evenly apart. Bake for 10 to 13 minutes until the edges of the cookies are crinkled and look set.