01 -
Heat the oven to 350°F. Line 2 baking sheets with parchment paper and set aside.
02 -
In a medium bowl, whisk flour, cornstarch, baking powder, baking soda, and salt until evenly combined.
03 -
Beat softened butter and sugar in a large bowl for about 2 minutes, until lighter and fluffy. Beat in the egg, egg yolk, and vanilla until smooth.
04 -
Add the dry ingredients to the butter mixture on low speed. Mix just until a soft dough forms and no dry flour remains.
05 -
Divide dough into 5 equal portions. Knead 1 tablespoon dry Jello mix into each portion, using a different flavor for each. Add a tiny drop of matching gel color if desired.
06 -
Roll dough into 1 1/2 tablespoon balls. Roll lightly in granulated sugar if using. Place on the prepared sheets and chill for 20 minutes.
07 -
Arrange chilled dough balls 2 inches apart. Bake for 9-11 minutes, until the tops look set and the edges are barely firm.
08 -
Let cookies cool on the baking sheet for 10 minutes, then transfer to a wire rack to cool completely.