Cream Butter & Sugar: In a large bowl, beat together the softened butter and granulated sugar until the mixture is light and fluffy (2-3 minutes with an electric mixer).
Add the Egg: Beat in the egg until it's fully incorporated. Scrape down the sides of the bowl.
Whisk Dry Ingredients: In a separate medium bowl, whisk together the all-purpose flour, baking powder, salt, and both packages of Jello powder. Ensure the Jello powder is evenly distributed.
Combine Wet & Dry: Gradually add the dry ingredients to the wet ingredients, mixing on low speed (or stirring by hand) until just combined. Do not overmix. The dough will be thick and might look crumbly initially.
Chill the Dough: Cover the bowl with plastic wrap and refrigerate the dough for at least 30 minutes. This makes it easier to handle and helps cookies hold shape.
Preheat Oven & Prep: While chilling, preheat your oven to 375°F (190°C). Line baking sheets with parchment paper or silicone baking mats.
Shape the Cookies: Take spoonfuls of the chilled dough and roll them into balls, about 1 inch in size. Optionally, roll these balls in a little extra granulated sugar for a sparkly finish. Place balls about 2 inches apart on prepared baking sheets.
Bake! Bake for 8-10 minutes, or until edges are lightly set but centers still look soft. They should not brown much.
Cool: Let the cookies cool on the baking sheets for a couple of minutes before carefully transferring them to a wire rack to cool completely.