In a medium saucepan, melt the butter and chocolate, stirring frequently. Once melted, set aside to cool slightly.
In a medium bowl, whisk together the flour, cocoa powder, baking powder and salt.
In the bowl of an electric mixer, beat the sugar, eggs and vanilla on low speed until well combined.
Add the cooled chocolate mixture and blend until just combined.
While mixing, add the flour mixture slowly and blend until just combined.
Scrape down the sides of the bowl, then cover the dough and refrigerate for a minimum of 2 hours. If making the dough a day ahead, let sit at room temperature for 30-45 minutes before shaping.
Preheat oven to 325 degrees F and prepare two baking sheets with cooking spray. Scoop out dough one tablespoon at a time and roll into balls. Place the balls two inches apart on the baking sheets, then gently press an indention into the center of each with your thumb (wetting your thumb helps keep cookie dough from sticking).
Bake cookies for approximately 12 minutes. Remove from oven. If the indentions have filled up during baking, press the center of the cookie back down with the back of a spoon. Allow cookies to cool completely.