Preheat the oven to 300°F. Grease the miniature Bundt pans with butter and dust with cocoa powder. I find using my fingers helps to make sure the butter gets into all the nooks and crannies of the mold.
Whisk the dry ingredients – flour, baking soda, cocoa powder, and salt – together in a large bowl.
In the top pan of double boiler, melt the butter with the chopped chocolate bars, stirring until smooth. If you do not have a double boiler, you can use a heat-safe bowl set into a sauce pan of simmering water (just be very careful, as the bowl can become hot).
Transfer the melted chocolate and butter mixture into the bowl of a stand mixer. Stir in the bourbon and sugar. Beat in the dry ingredients a half a cup at a time until just incorporated. Beat in the eggs one at a time, followed by the vanilla extract.
Fill the miniature cake molds three-fourths of the way full. Bake for 30-40 minutes, until a toothpick can be inserted into the cakes and come out clean. Let cool in the molds for 10 minutes, then carefully invert the pans onto racks and remove the cakes. Allow the cakes to cool completely (at least an hour), or chill overnight, covered in plastic wrap.