Grease and flour 10-inch tube pan with removable bottom.
In small pan on low heat, melt bars stirring frequently until blended.Set aside.
In medium bowl,sift together flour, baking powder and baking soda.
In large bowl, use electric hand mixer on medium low speed, then medium speed ,to beat together sugar and shortening until fluffy, about 5 minutes.Beat in eggs 1 at a time thoroughly incorporating each one. Beat in vanilla and Mars bar mixture until blended.Beat in a third of flour mixture.
Beat in half of the buttermilk.Beat in the remaining flour mixture. Beat in the remaining buttermilk.
Pour batter into prepared pan ,bake in preheated 325 oven 55 minutes or until tester comes out clean. Cool on wire rack 15 minutes.Turn out of pan on to serving plate.Cool completely.
For icing, melt bars and butter on low heat, stirring frequently until blended.Remove from heat. Cool 5 minutes. Add icing sugar, milk and vanilla. Whisk vigorously until smooth. Spoon over cake,allowing it to drip down sides.
For topping arrange Mars slices decoratively on top of cake.