Caramel Peanut Butter Chocolate Chip Gooey Bars

unsalted butter (1 stick), melted

large egg

light brown sugar, packed

vanilla extract

all-purpose flour

salt, optional and to taste

graham crackers (standard graham crackers, about 3-inches by 5-inches), chopped into 1/2-inch square pieces (about 1 heaping cup loosely piled pieces)

semi-sweet chocolate chips

sweetened condensed milk

creamy peanut butter