1. In a heatproof bowl set over a pan of barely simmering water (bottom of bowl should not touch water), stir chopped chocolate and butter often until smooth, about 5 minutes. Remove bowl from over water and, with a whisk or a mixer on medium speed, beat in sugar, egg, and vanilla until blended.
2. In a medium bowl, mix flour, baking soda, baking powder, and salt. Add to chocolate mixture and stir until well blended. Stir in chocolate chips. Cover and chill dough until firm, about 1 hour.
3. Drop dough in 1-tablespoon portions, 3 inches apart, onto buttered 12- by 15-inch baking sheets.
4. Bake in a 350° oven just until cookies are set at the edges, 7 to 10 minutes; if baking more than one pan at a time, switch pan positions halfway through baking.
5. Let cookies cool for 5 minutes on sheets, then use a wide spatula to transfer to racks to cool completely.
Note: Nutritional analysis is per cookie.