Line a 13- x 9- x 2-inch baking pan with foil, extending the foil over edges of pan. Lightly butter foil; set aside.
In a large bowl, combine the semisweet chocolate chips, milk chocolate chips, marshmallow creme, walnuts, and vanilla.
In a heavy large saucepan, stir together the sugar, evaporated skimmed milk, and butter. Bring to boil over moderate heat, stirring constantly with a wooden spoon to dissolve sugar. (Avoid splashing mixture onto side of pan.) Lower the heat to moderately low. Stir and boil for 1 2 minutes. Remove from heat.
Carefully pour the boiling mixture over the chocolate mixture. Stir until the chocolate is melted and the mixture is creamy and well combined. Quickly turn the fudge into prepared pan. While fudge is warm, use the tip of a small knife to score into 1-inch squares.
When fudge is firm, lift it out of pan; cut into squares. To store, cover tightly and refrigerate.
Cutting Fudge: By lining the pan with foil, you can remove the fudge quickly and easily. When the candy is firm, grasp the foil and lift the block of fudge out of the pan. Remove the foil and place the candy on a cutting board.
Using a long-bladed, serrated knife, such as a bread knife, cut along the lines scored in the fudge. To get a smooth even cut, place one hand on the knife handle and the other on the top of the blade, then press down evenly until the knife cuts through the candy.
When Mamie Eisenhower came to the White House, she brought along a recipe for fudge that called for marshmallow creme. Her candy was so creamy the president reportedly called it the "million dollar" fudge.