1. Preheat oven to 375°.
2. Coat a 13 x 9-inch metal baking pan with cooking spray; line bottom of pan with parchment paper.
3. Combine graham cracker crumbs, chocolate chips, and walnuts in a large bowl. Add vanilla and milk, stirring until well blended. Spread mixture into prepared pan. Bake at 375° for 20 to 25 minutes or until edges are golden brown. Cool completely in pan on a wire rack.