In a large bowl set over (but not touching) barely simmering water, stir chopped semisweet chocolate and butter often until chocolate is melted and mixture is smooth, about 5 minutes. Remove bowl from over water and, with a whisk or a mixer on medium speed, beat in the sugar, egg, and vanilla until well blended.
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.
Drop dough in 1-tablespoon portions 3 inches apart onto buttered 12- by 15-inch baking sheets.
Bake in a 350° regular or convection oven just until cookies are set on the edges but still slightly soft in the center, 7 to 10 minutes; if baking two sheets at once in one oven, switch their positions halfway through baking. Let cookies cool for 5 minutes, then use a wide spatula to transfer to racks to cool completely.