Using an electric mixer, cream the sugar, butter and salt at high speed until well blended, about 3 minutes. Scrape down the bowl, add the egg, egg white and vanilla and beat for 1 minute.
Beat in the cocoa at medium speed until thoroughly blended. Add the flour, baking powder, orange peel and white chocolate chips and mix at low speed just until incorporated.
Preheat the oven to 350 degrees . Line 2 cookie sheets with parchment.
Roll the dough into 1 1/2-inch balls; roll in confectioners sugar. Place on the prepared pans, spacing evenly. Bake 1 pan at a time (refrigerate the other pan) until the centers are springy, about 15 minutes. Let cool on the pans for 2 minutes; transfer to racks to cool completely.
Place two-thirds of the semisweet chocolate in a microwavable bowl. Microwave at medium power for five to six 30-second intervals, stirring in between, until smooth. Add the remaining chocolate; let stand for 5 minutes. Stir until smooth.
Dollop some chocolate on top of each cookie; sprinkle with orange peel. Let stand until set, about 1 hour.