1. Make cookies: Position rack in center of oven and preheat to 375°F. Line 3 large baking sheets with parchment paper.
2. Put flour, baking powder and salt in medium bowl. Stir with wire whisk to blend. Set aside.
4. In bowl of electric mixer, using paddle attachment, beat butter and sugars on medium speed for about 5 minutes, or until lighter in color and fluffy. Beat in vanilla extract. Scrape down sides of bowl. Add eggs, one at a time, beating for 30 seconds after each addition and stopping to scrape down sides of bowl as necessary. Add melted chocolate and beat until blended. Reduce speed to low and add half of flour mixture. Beat just until absorbed. Remove bowl from mixer stand and, using large rubber spatula, fold in remaining flour, then chocolate chunks and nibs. Let batter rest for about 10 minutes to thicken slightly.
5. Drop dough in 2-tablespoon-sized mounds (preferably using small ice cream scoop) onto one lined baking sheet, making sure mounds do not touch each other. Lightly cover with plastic wrap and chill for about 2 hours (or up to 8 hours), until very firm.
6. Remove 9 chilled mounds from refrigerator and transfer to another lined baking sheet, spacing cookies about 2" apart. Bake cookies for 8 to 10 minutes, or until tops are puffed and slightly cracked and have a dull sheen, but are not wet looking. Slide paper off baking sheet and transfer to wire rack. Allow cookies to cool completely before detaching from paper. Do not overbake or cookies will be dry.
7. Repeat previous step with remaining cookies, keeping last batch on original baking sheet and spacing cookies 2" apart.
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