Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a medium bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt.
In a large bowl, beat the butter, granulated sugar, and brown sugar until smooth and fluffy (about 2 minutes).
Beat in the egg and vanilla until well combined.
Slowly mix in the dry ingredients until just incorporated.
Stir in chopped Golden Oreos and M&Ms, reserving a handful to press on top. You can chill the dough for 30 minutes if you want thicker cookies, but you can also bake immediately.
Roll or scoop 1 ½–2 tablespoon portions of dough onto the prepared baking sheet. If your dough is too crumbly, you can gently press it together with your hands or pack it into the cookie scoop for a smooth, compact shape.
Press a few extra M&Ms and Golden Oreo chunks into each dough ball.
Bake for 9–11 minutes, until edges are set but centers are still soft.
Let the cookies cool completely on the baking sheet. Use a rubber spatula to lift them gently if needed.