In a bowl, mix the brown sugar and granulated sugar into the browned butter until smooth.

Add the egg and vanilla extract and mix well.

In a separate bowl, whisk together the flour, baking soda, baking powder, salt, cornstarch, and cinnamon (if using).

Add the dry ingredients into the wet ingredients and mix until just combined (don’t overmix).

Stir in the rolled oats until evenly spread through the dough.

Chill the dough in the freezer for 10 minutes.

Scoop dough onto a baking sheet.

Bake at 350°F (177°C) for 7–8 minutes, until edges look set but centers still look soft.

Let cool on the tray for a few minutes before eating so they firm up.