Dulce de Leche Coconut Chocolate Chip Bars

unsalted butter (1 stick), melted

light brown sugar, packed

vanilla extract

all-purpose flour

salt, optional and to taste

white chocolate chips (butterscotch or peanut butter chips may be substituted)

loosely packed sweetened shredded coconut

semi-sweet chocolate chips

half of one 14-ounce can dulce de leche (make your own, or storebought; sweetened condensed milk may be substituted but the flavor/texture of bars will change)