Place the butter in a microwave-safe bowl. Microwave on high until melted, 30-40 seconds.

Transfer the melted butter to a large mixing bowl and add the brown sugar. Whisk together until combined.

Add the egg and vanilla extract. Continue whisking until the mixture is thoroughly combined.

Add the flour, baking soda, cornstarch, cinnamon, and salt to the bowl. Use a wooden spoon to stir vigorously until a soft and smooth dough forms.

Cover the bowl with plastic wrap and place in the refrigerator to chill for at least one hour.

Line two baking sheets with silicone baking mats or parchment paper. Set aside.

Preheat the oven to 350°F.

Remove dough from refrigerator. Form the dough into 20 balls and place on the prepared baking sheets.

Bake the cookies until they are just slightly golden brown on the edges and puffy on top, 11-13 minutes.

Check to see that cookies are done. Remove from oven or add time as needed.

Allow the cookies to cool for 5 minutes on the baking sheets. Transfer the cookies to cooling racks to cool completely.

Serve while still slightly warm or completely cooled. Leftover cookies may be stored in an airtight container at room temperature for 3 days, or 5 days in the refrigerator.