How to Make It
Combine the sugar, butter, egg, and vanilla in the bowl of a stand mixer. Beat on medium speed until fluffy, about 3 minutes. Add the flour, cornstarch, baking soda, and salt; beat until just combined.
Pulse the Froot Loops in a food processor until crumbled. Remove 1 cup of crumbles, then continue pulsing the remaining pieces until very finely ground and powdery. Fold the crumbled pieces into the dough and stir to combine. Pour the Froot Loops powder into a small bowl.
Form the dough into 15 balls, then roll each ball in the Froot Loops powder. Place on two parchment-lined baking sheets and refrigerate for at least 1 hour, or overnight.
Heat oven to 350°F. Bake cookies until puffy and just barely golden, about 11 minutes. Let cool for 30 minutes, then serve.