Chocolate Glaciers

Whisk flour, cocoa, baking powder, and salt in a medium bowl until blended. In another bowl, beat butter, granulated sugar, and brown sugar with mixer until just blended, about 1 minute. Beat in eggs one at a time, then vanilla. With mixer on low, beat in flour mixture until just incorporated (dough should be very sticky). Chill dough 30 minutes. Heat oven to 350 degrees F. Line several cookie sheets with parchment or nonstick liners. Sift confectioners' sugar onto a large plate. Put about 8 heaping teaspoons of dough onto the plate. Roll each clump in sugar to lightly coat, then shape into a ball; re-roll balls in sugar until heavily coated. Repeat with remaining dough. Place balls 2 inches apart on prepared cookie sheets. Bake one sheet at a time for 11 minutes or until tops are cracked and cookies are just set around edges but still soft in the middle.