In a large bowl, beat the softened butter with the granulated sugar until the mixture turns light and fluffy. Using an electric mixer makes this easier and faster, but you can also do it by hand with a sturdy whisk. This step is important because it traps air and makes the cookies soft instead of dense.
Once the butter and sugar are creamy, crack in the egg and beat again until fully mixed. Make sure to scrape down the sides of the bowl so everything blends together smoothly. The egg gives the dough structure and helps hold the cookies in shape while baking.
In a separate medium bowl, whisk together the flour, baking powder, salt, and both packs of Jello powder. This step ensures the Jello flavor spreads evenly throughout the cookies. Be careful not to skip mixing well, or some cookies may taste stronger than others.
Slowly add the dry mixture into the wet ingredients, stirring on low speed. The dough will feel thick and may look crumbly at first, but keep mixing until it comes together. Avoid overmixing as it can make the cookies tough instead of tender.
Cover the bowl with plastic wrap and place it in the fridge for at least 30 minutes. Chilling is very important because it makes the dough easier to roll and prevents the cookies from spreading too much while baking. Cold dough bakes into neater shapes.
Scoop out small portions of the dough and roll them into balls about one inch in size. If you like, roll each ball in extra granulated sugar to give the cookies a shiny look. Place them on lined baking sheets about two inches apart so they have space to expand.
Preheat the oven to 375°F (190°C). Bake the cookies for 8–10 minutes until the edges are set but the centers still look slightly soft. They should not turn brown, as that would make them dry. Let the cookies cool for a couple of minutes on the baking sheet, then transfer them to a wire rack to cool completely.