Soft and Chewy Smores Cookies

unsalted butter, softened

light brown sugar, packed

granulated sugar

large egg

vanilla extract

all-purpose flour

cornstarch

baking soda

salt, optional and to taste

coarsely chopped graham crackers, about 4 full-size cracker sheets

(one 12-ounce bag) semi-sweet chocolate chips

Marshmallow Bits (regular mini-marshmallows may be substituted; if using, shelter them with some dough and don’t let them bake directly on baking sheet to prevent scorching)