Preheat oven to 375 degrees F or 350 degrees F. I used 350F.
Line a 13x9 inch metal pan with foil and spray with baking spray. If making a half batch, use an 8 inch square metal pan.
Mix the sifted flour, salt and baking soda in a small bowl and set aside.
In mixing bowl, using an electric mixer, cream butter and sugar until light. Add eggs one at a time, beating well after each addition. Mix milk with vanilla. Add flour and milk alternately to creamed mixture, beating well. Take ⅓ of the mixture (I recommend slightly more than ⅓) and put in a separate bowl with the melted chocolate and walnuts. Stir until mixed. Spread the chocolate mixture in the pan. Lay the graham crackers over the chocolate mixture, then mix the chocolate chips in with the remaining ⅔ (white) batter. Spread that over the graham crackers.
Bake for 20 to 25 minutes. My half batch took 23 minutes at 350F. Cool completely, then lift from the pan and cut into 32 bars or however many you prefer. If you plan on dipping the bars, cut into long wedges rather than squares.
For chocolate dipped bars, melt the butter in a 2 cup liquid measuring cup (Pyrex), add the chopped chocolate and stir until it melts as much as it possibly can from the heat of the butter, then microwave at 10 second intervals, stirring at each interval, until it’s smooth and completely melted. Dip the bars right in the cup. After they’ve set, slice them into squares.