Preheat oven to 350 degrees F and line an 8x8 pan with parchment paper. Combine the flour, cornstarch, baking powder, and salt in a medium bowl. Whisk to mix and set aside.

In a large bowl, beat together the butter and sugars. Add the eggs and vanilla and beat to combine.

Add the flour mixture to the butter mixture and beat to combine.

Fold in the chocolate chips, caramel, and pretzels. Reserve a small amount of each ingredient for sprinkling on top of the cookie bars, if desired.

Spread the batter into the prepared pan. Top with reserved chocolate chips, caramel, and pretzel pieces.

Bake in the preheated oven for 30 to 35 minutes, until the center of the cookie bars is set.

Remove from oven and allow to cool to room temperature before slicing.