Peanut Butter Sandwich Cookie Bars

unsalted butter (1 stick), melted

large egg

light brown sugar, packed

heaping 1/3 cup creamy peanut butter

vanilla extract

all-purpose flour

salt, optional and to taste

bag peanut butter chips, divided (3/4 cup stirred in and 1 cup melted)

half of one 14-ounce can sweetened condensed milk (just eyeball it)