In the bowl of a stand mixer fitted with a paddle attachment, cream butter, brown sugar, and granulated sugar together on medium speed, beating until light and fluffy about 3 minutes. Add in egg and vanilla and mix until incorporated on low speed. Turn the mixer up to medium-high and mix for 1 minute.

Scrape down the sides of the bowl if needed. Add the cornstarch, baking soda, and salt. Mix for 30 seconds then add the flour 1 cup at a time. Mix until fully incorporated.

Add the chocolate pieces and either beat for a few seconds on low speed or fold in by hand.

Preheat the oven to 350 degrees F. Line a baking sheet with parchment paper.

Cut through the middle of the croissant horizontally with a sharp serrated knife, leaving the back uncut, and place on the baking sheet. Repeat with each croissant.

Using about 1/2 a cup of cookie dough per croissant, take some of the dough, and press a thin layer evenly across the inside of the cut croissant, and press it closed.

Add the rest of the dough from the 1/2 cup of dough to the top of each end of the croissant. It helps to flatten the dough with your hands and then press on the croissant. Repeat with the croissants until all the cookie dough is used.

Place the croissants on the baking sheet and bake for 10-12 minutes, until the cookie dough is slightly golden.

Let cool slightly, and enjoy!