In a large bowl, whisk together the flour, baking soda, cornstarch, cinnamon, and salt. Set aside.

In a medium bowl, whisk the melted butter and brown sugar together until no brown sugar lumps remain. Whisk in the egg and vanilla. Pour the wet ingredients into the dry ingredients and mix together with a silicone spatula or large wooden spoon until completely combined. The dough will be very soft, yet thick.

Cover the dough and refrigerate for at least 3–4 hours, or up to 3 days. Chilling is mandatory.

Take the dough out of the refrigerator and allow to slightly soften at room temperature for 15 minutes if it chilled for longer than 4 hours.

Preheat the oven to 325°F (163°C). Line large baking sheets with parchment paper or silicone baking mats. Set aside.

Pour the granulated sugar into a small bowl. Scoop and roll dough into a ball, about 1.5 Tablespoons (30–35g) in size, then roll in the sugar. Place 3 inches apart on the prepared baking sheets (8–9 cookies per baking sheet).

Bake for 14–16 minutes, until the edges of the cookies are set and small cracks are appearing. The cookies will be puffy and still appear very soft in the middle, but they will deflate slightly and continue to set as they cool. Remove from the oven and allow to cool on the baking sheet for 10 minutes before transferring to a cooling rack to cool completely.

Cover cooled cookies tightly and store at room temperature for up to 1 week.