In a large bowl, whisk together flour, cornstarch, baking powder, baking soda, and salt until well combined.

Carefully spoon the flour mixture into the bottom of your mason jar. Tamp it down to pack the flour as much as possible. Aim for straight edges against the glass for a visually pleasing layer, but don’t worry if it’s a bit uneven in the middle. If there’s excess flour on the sides, gently wipe it down with a paper towel or pastry brush.

Layer the dark brown sugar over the flour, pressing firmly. Follow with a layer of light brown sugar, and then top with granulated sugar, ensuring each layer is packed and even.

Add your chosen mix-ins, filling to the very top of the mason jar. The goal is to pack everything tightly to prevent shifting during transportation.

Secure the lid tightly. Attach a Free printable instruction tag (provided above) to the jar with baker’s twine or ribbon.

Preheat your oven to 350°F. Let the melted butter cool until it's no longer warm to the touch.

In a separate bowl, stir together the cooled butter, egg, and vanilla extract.

Pour all the ingredients from the jar into the bowl. Stir well until completely combined, ensuring any clumps are broken up.

Drop the cookie dough by rounded 1 ½ tablespoon-sized scoops onto an ungreased cookie sheet. Bake for 9-11 minutes.

Allow the cookies to cool on the baking sheet for 5 minutes before transferring to cooling racks.