Using an electric mixer, cream butter and sugar in a medium-sized bowl. Add flour and walnuts and combine to make a crumb mixture. Reserve 3/4 to 1 cup for the topping. With your hands, a tart tamper, or the bottom and side of a dry measuring cup, press the remainder into an 8-inch square pan. Bake for 12-15 minutes, or until light brown. Remove from oven and place on a wire rack.
Clean or wipe the medium-sized bowl and beaters used for crumb mixture. Whisk sugar with softened cream cheese until smooth. Add egg, vanilla, milk, and lemon juice. Beat on low until well mixed. Pour the mixture in the middle of the baked crust. While wearing oven mitts, hold each side of the pan and move the batter side to side until it's even across and up the side of the crust. Sprinkle the reserved crumb mixture over the top of the batter. Bake for 25 minutes.
Cool on a wire rack until room temperature. Place in refrigerator until cold. Cut into 2-inch squares. Share the magic!