Brown the butter by cooking it over medium heat in a medium saucepan until it’s amber to brown in color, about 5 minutes. Swirl the pan in the last minute to see the color changes. Butter will go through stages of hissing and sputtering until the water cooks off, at which point the browning occurs. Butter will smell nutty and aromatic.
Pour the browned butter into a large mixing bowl, scraping the bottom with a spatula to transfer any browned bits for flavor. Wait a moment before adding the egg.
Add the egg, light brown sugar, granulated sugar, and vanilla, then whisk to combine.
Add the all-purpose flour, cornstarch, baking soda, and salt. Stir until combined. The dough may seem crumbly yet oily.
Fold in the TURTLES® Minis and semi-sweet chocolate chips. Ensure everything is evenly distributed.
Using a large cookie scoop, form about 11 to 12 mounds of dough, roll into balls, and flatten halfway. Place a halved TURTLES® Mini on top of each mound and add more chocolate chips for a pop of chocolate.
Place mounds on a large plate or tray, cover with plastic wrap, and refrigerate for at least 4 hours, up to 5 days.
Preheat oven to 350°F. Line a baking sheet with a Silpat or spray with cooking spray. Place dough mounds on the sheet, spaced at least 2 inches apart, and bake for 12 to 13.5 minutes until edges are set and tops are glossy.
Cool cookies on the baking sheet for about 10 minutes before serving. Don't use a rack; they firm up as they cool.