Preheat your oven to 350°F (175°C) and line two large baking sheets with parchment paper.

In a medium bowl, whisk together the all-purpose flour, cornstarch, baking soda, baking powder, and salt. Set aside.

In the bowl of a stand mixer with the paddle attachment, or in a large bowl with a hand mixer, beat the softened butter until smooth. Add the granulated sugar and light brown sugar and beat on medium-high speed for 2-3 minutes until light and fluffy. Scrape down the sides of the bowl.

Add the creamy peanut butter and beat again until fully combined and smooth.

With the mixer on low, add the room temperature egg, vanilla extract, and optional molasses. Mix just until combined, being careful not to overmix. Scrape down the bowl.

Turn the mixer to its lowest speed and gradually add the dry ingredient mixture. Mix only until the last streaks of flour have disappeared. The dough will be soft.

Using a medium cookie scoop (about 1.5 tablespoons), portion and roll the dough into smooth balls. Place them 2 inches apart on the prepared baking sheets.

Dip a fork in granulated sugar and gently press down on top of each dough ball, then press again from the opposite direction to create a crisscross pattern.

Bake for 10-12 minutes, until the edges are set and lightly golden but the centers still look slightly soft and puffy. Do not overbake.

Let the cookies cool on the baking sheet for 5 minutes to set up, then transfer them to a wire rack to cool completely.