Preheat oven to 325 degrees and spray a 10-cup Bundt pan with baking spray.
In a heavy-bottom saucepan, melt 1/2 cup of butter and 8 chopped Milky Way candy bars over low heat, stirring frequently until smooth. Remove from heat.
Using an electric mixer, beat the remaining 1/2 cup butter and sugar until light and creamy, about 7 minutes.
Add eggs, one at a time, beating well after each addition and scraping down the sides of the bowl.
Stir together flour and baking soda. Add flour mixture, alternating with buttermilk. Beat just until mixed evenly.
Add melted candy/butter mixture and vanilla and mix well.
Pour into prepared pan. Bake for 65 to 70 minutes. Let cool 10 minutes and then remove from pan.
To make frosting, melt butter and Milky Way bars in a heavy bottomed saucepan, whisking until smooth. Transfer mixture to a bowl and beat in remaining ingredients.
Spread frosting on cake.