Line a plate with small piece of parchment paper. Use a small cookie scoop or teaspoon to portion out the biscoff spread into 2 teaspoon round mounds. Place them onto the lined plate and freeze while you prepare the cookie dough.
In a medium bowl, stir together flour, cornstarch, baking powder, baking soda, and salt. Set aside.
In a large mixing bowl, use an electric hand mixer or stand mixer to beat softened butter until creamy. Add in the brown sugar and granulated sugar and beat until fluffy, about 3 minutes. Add in the egg and vanilla extract and beat again until combined.
Add the flour mixture into the wet mixture. Mix on low speed until mostly combined but with some dry spots of flour remaining. Add in the chopped white chocolate and fold together with a silicone spatula.
Cover and chill the dough in the fridge for at least 1 hour until it firms up.
Preheat oven to 350°F and line a baking sheet with parchment paper. Set aside.
Use a large ice cream scoop to scoop out a ball of cookie dough. Release it into your hand and use your fingers to hollow out a small cavity in the middle of the dough.
Place one of your frozen biscoff balls into the middle of the cookie dough. Press the dough together to seal the opening. Place the dough with the seam side down onto your lined baking sheet.
Repeat with the remaining dough and biscoff balls, leaving at least 2” in between each dough ball for spreading. Press a few white chocolate chunks on the surface of each cookie dough ball (optional).
Bake for 12-13 minutes for small cookies or 14-15 minutes for large cookies until the edges are set but middle still looks slightly underbaked. They will continue cooking on the baking sheet out of the oven.
Sprinkle cookies with flaky salt and let cool on the baking sheet for about 10 minutes before transferring to a cooling rack to cool completely or enjoy while warm!