Put the chocolate in a small microwave-safe bowl and microwave in 30-second intervals, stirring, until melted, about 2 minutes. Let cool slightly. Whisk the flour, baking powder and salt in a medium bowl.
Beat the butter and sugar in a large bowl with a mixer on medium-high speed until light and fluffy, 3 to 5 minutes. Beat in the egg until smooth. Beat in the melted white chocolate, then the vanilla. Reduce the mixer speed to low and beat in the flour mixture in three additions until just combined. Divide the dough between 2 sheets of plastic wrap and pat into disks; wrap and refrigerate until firm, at least 2 hours or overnight.
Position racks in the upper and lower thirds of the oven; preheat to 350 degrees F. Line 2 baking sheets with parchment paper. Roll out the dough on a floured surface until 1/4 inch thick. Cut out cookies using a 3- to 3 1/2-inch cookie cutter. Arrange 1 1/2 inches apart on the prepared pans. Freeze until firm, about 10 minutes. Gather the scraps and refrigerate until firm, then reroll once, cut out more cookies and freeze.
Bake, switching the pans halfway through, until the cookies are firm and the edges are light brown, 12 to 15 minutes. Let cool 5 minutes on the pans, then transfer to racks to cool completely.
Decorate the cookies with royal icing. Sprinkle with nonpareils. Let set, about 1 hour.