Preheat oven to 350 degrees F (175 degrees C). Grease and flour 12 muffin cups.
In a large bowl, cream together the butter and sugar until light and fluffy. Beat in the eggs one at a time, then stir in the vanilla. Combine the cocoa, flour, baking soda and salt; stir into the creamed mixture alternately with the buttermilk. Fold in bacon.
Fill muffin cups 2/3 full. Bake for 20 to 25 minutes in preheated oven, or until a toothpick inserted into center of a cupcake comes out clean.