Preheat oven to 325 degrees F. Grease and flour three 9-inch cake pans.
In a double boiler, melt the candy bars with 1/2 cup butter. As the candy bars and butter are melting, they will begin to look almost curdled, but it will eventually come together into a smooth consistency. Remove from heat and let cool.
In a large bowl, use an electric mixer to combine the remaining 1 cup of butter with the sugar. Mix in the beaten eggs and cooled chocolate mixture.
Sift the flour and baking soda together. Alternately mix in flour and buttermilk to the batter, blending well. Add the vanilla.
Divide the batter between the three cake pans, filling each no more than halfway. Bake 30 to 40 minutes, or until toothpick inserted in the center comes out clean. If you have extra batter, make a few cupcakes!
To prepare the frosting: Combine the sugar and evaporated milk in a medium saucepan. Cook to a soft-ball stage. Remove from heat and add butter, marshmallow cream and chocolate chips, stirring until all have melted.
Place one layer of the cake on a platter. Frost the top of that layer, letting the frosting spill over the sides a bit. Add another layer of cake on top of the first. Frost that one the same way. Add the final layer of cake on top and use the rest of the frosting. Note that the sides of the cake will be "naked," and the frosting is just spread on top of each layer.
Notes
Tips for the SOFT BALL STAGE: at this temperature, sugar syrup dropped into cold water will form a soft, flexible ball. If you remove the ball from water, it will flatten like a pancake after a few moments in your hand.