In a mixing bowl, combine butter and sugar. Then add in eggs, vanilla, and sour cream until mixed.
In a separate bowl, combine flour, cocoa powder, baking powder, baking soda, and salt.
Pour about half of the flour mixture into the butter mixture and slowly combine.
Mix the buttermilk in. Then add the remaining flour mixture to the bowl.
Pour cake mixture into a greased bundt pan and bake at 350 degrees for 45-50 minutes.
Once the cake is cooled, combine powder sugar, milk, and vanilla until smooth.
Then pour it over the top of the cake. You can save some of the glaze to pour over each slice of cake as you serve it.