In the top of a double boiler or in a bowl set over a pot of simmering water (be sure the bottom of the bowl is not touching the water), heat chocolate over simmering water, stirring occasionally, until melted. Let cool slightly.
12 ounces chopped 60% cacao semisweet chocolate baking bars
In the bowl of a stand mixer fitted with the paddle attachment, beat butter and sugars at medium speed until fluffy and pale yellow, about 2 minutes, stopping to scrape sides and bottom of bowl. Add eggs and vanilla, and beat until well combined, about 1 minute, stopping to scrape sides and bottom of bowl. Add melted chocolate, and beat at low speed until well combined. Scrape sides and bottom of bowl, and beat for 30 seconds.
½ cup unsalted butter, 1 cup brown sugar, ¼ cup granulated sugar, 3 large eggs, 1 teaspoon vanilla extract
In a medium bowl, whisk together flour, cocoa, baking powder, and salt. Add flour mixture to butter mixture, and stir gently just until combined. (Do not overmix.) Loosely cover with plastic wrap, and refrigerate for 30 to 45 minutes.
1 ¼ cup all-purpose flour, ¼ cup unsweetened cocoa powder, 1 teaspoon baking powder, ½ teaspoon kosher salt
Preheat oven to 350°F (180°C). Line 2 baking sheets with parchment paper.
Scoop dough by 2 tablespoonfuls, and roll into balls. Place at least 2 inches apart on prepared pans.
Bake for 11 to 13 minutes. Let cool completely on pans.