Heat oven to 350° F.
Place a strip of parchment paper in an 8"x8" baking dish, large enough so there is overhang on both ends of the dish (a sling).
Spray the baking dish/paper with baking spray.
Break up the graham crackers into a food processor. Pulse several times until they are fine crumbs (you can also use a large plastic zip-close baggie and smash the crackers with a rolling pin).
Add the crumbs to a mixing bowl.
Pour in the sweetened condensed milk and vanilla. Mix until fully incorporated.
Add the chocolate chips and walnuts. Mix to combine. The dough will be very thick and sticky.
Press the dough evenly into the pan using your fingers.
Bake at 350° for 25-30 minutes, or until the brownies are lightly golden on top and a toothpick comes out clean.
Let cool for 15 minutes in the baking dish, then use the ends of the parchment paper to lift the brownies out and onto a wire rack to finish cooling.
Cut into 16 squares.