Step 1Heat the oven to 375 degrees. Line 2 baking sheets with parchment paper.
Step 2Blitz the purple sparkling sugar in a food processor until powdery, then add the powdered sugar and pulse to combine (alternatively, process the purple sugar in a spice grinder to powder, then whisk into the powdered sugar). Transfer the purple powdered sugar to a shallow bowl.
Step 3Sift the flour, cocoa powder and maca powder in a medium bowl. In a large bowl, combine the chocolate, both sugars, the sour cream, butter, salt, baking powder, baking soda, vanilla and eggs and whisk until smooth. Fold in the flour mixture with a rubber spatula until smooth.
Step 4Use a ½-ounce ice cream scoop or one tablespoon to drop a dough ball into the purple sugar and roll to coat. Transfer to a prepared baking sheet. Repeat with the remaining dough and purple sugar, arranging the balls 2 inches apart on the sheets.
Step 5Bake, rotating the baking sheets from front to back and top to bottom halfway through, until the cookies are firm at the edges but still soft in the middle, 8 to 9 minutes.
Step 6Let the cookies cool on the baking sheets for 1 minute before transferring to a wire rack to cool completely. Sprinkle cookies with more purple sugar, if you like, before serving.