How to Make It
Preheat oven to 350°F. Whisk together egg and lemon curd in a large bowl until thoroughly blended. Add whipped topping, stirring well to combine. Stir in cake mix until combined.
Place granulated sugar in a medium bowl. Place powdered sugar in a separate medium bowl. Shape dough into balls (about 2 tablespoons each). Working with 1 ball at a time, roll in granulated sugar until well coated. Transfer ball to powdered sugar, and coat well. Place balls 3 inches apart on 2 large rimmed baking sheets lined with parchment paper (8 cookies per baking sheet). (Remaining dough balls can sit, uncovered, at room temperature for up to 30 minutes.)
Bake in preheated oven until tops are set and cracked, 12 to 14 minutes (tops will feel soft). Cool on baking sheets 5 minutes. Transfer to wire racks, and cool completely, about 10 minutes.