Mix butter, flour, baking soda, nuts, brown sugar, oats, and salt. Reserve 1 ½ cup for topping. Spread the remaining mixture into greased jelly roll pan. Bake @ 350 for 10 minutes. Melt 2 Tbsp butter and chips. Stir until smooth then add milk. Spread carefully over baked layer. Coarsely chop M&Ms and sprinkle over fudge layer. Sprinkle the reserved crust mixture over top. Bake for 20 minutes more.