Whisk the flour, granulated sugar, cocoa powder, baking powder, and salt in a medium bowl. Whisk in the melted butter and eggs until combined, then stir in the white chocolate chips. Cover with plastic wrap and chill until firm, about 1 hour.
Preheat oven to 350 degrees. Line 2 baking sheets with parchment paper. Put the confectioner's sugar in a shallow bowl. Roll tablespoonfuls of the dough into balls, then roll in the confectioner's sugar until well coated. Place 1 inch apart on the baking sheets.
Bake until the cookies are puffed and the tops are cracked, about 10 minutes. Let cool 2 minutes on baking sheets, then transfer to a rack to cool completely. Store in a airtight container up to 1 week.