Preheat oven to 350 degrees F
Place butter in a large, microwave-safe mixing bowl then microwave it just until butter is melted and warm (45 seconds - 1 minute). Add granulated sugar to the warm butter, then stir constantly for 2 minutes to help sugar dissolve. (Not all the sugar will dissolve). Set aside and allow butter to cool for about 10 minutes while you measure the dry ingredients.
In a separate bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt. Set aside.
Once butter mixture has cooled to room temperature, add brown sugar and stir well to eliminate lumps.
Add egg, oil, and vanilla extract to wet ingredients. Stir until smooth.
Add half of the flour mixture to the wet ingredients and mix well. Add the other half and mix until fully incorporated.
Line a large cookie sheet with parchment paper. Use a 1/4 measuring cup to scoop out the dough. Level off the dough and keep it a flat disc shape (about 1 inch - 1.25 inches tall); don't shape the dough into a ball. Place cookie dough discs on cookie sheet at least 3 inches apart.
Gently press rainbow sprinkles or coarse sugar into the tops of cookies (optional)
Bake for 12-15 minutes, just until the edges have slightly browned. Remove from oven and cool cookies undisturbed on tray for at least 10 minutes before serving (they will fall apart if removed too early). Enjoy!