Melt butter and 8 ounces of chocolate chips together in a microwave-safe bowl, stirring every 30 seconds until smooth. Let cool for 10 minutes.
In a large bowl, beat together sugar, eggs, and vanilla until thick and pale, about 2-3 minutes.
Stir the cooled chocolate mixture into the egg mixture until well combined.
In a separate bowl, whisk together flour, cocoa powder, baking powder, and salt.
Fold dry ingredients into chocolate mixture until just combined, then stir in remaining 4 ounces chocolate chips.
Cover and refrigerate dough for at least 1 hour (or up to overnight) until firm enough to handle.
Preheat oven to 350°F and line baking sheets with parchment paper. Place powdered sugar in a shallow bowl.
Roll chilled dough into 1½-inch balls and generously coat in powdered sugar.
Place cookies 2 inches apart on baking sheets and bake for 11-12 minutes until edges are set but centers still look slightly underbaked. Cool on baking sheet for 5 minutes before transferring to a wire rack.
