Preheat oven to 325°F degrees and position rack in the center. Butter and flour a mini Bundt pan.
Pour bourbon into a small bowl. Scrape seeds from vanilla bean into bourbon; stir to blend well (reserve scraped vanilla bean for another use like infusing sugar).
Whisk flour, cornstarch, baking powder, baking soda and salt in medium bowl to blend.
Using an electric mixer, beat butter, sour cream and sugar in large bowl until light and fluffy.
Add eggs and egg yolk to the butter mixture one at a time, beating until well blended after each addition. Beat in bourbon-vanilla mixture.
Add dry mixture in 2 additions alternating with milk in 1 addition, beating just until blended after each addition.
Split batter in half.
Add cocoa powder and milk to second batter and mix until incorporated.
Add the first layer of vanilla batter to mini bundt pan; Add a layer of cocoa batter. Add the last layer of vanilla. Mix together to create a marbled look.
Bake mini bundts for 20-25 minutes until tester inserted near center comes out clean.
Cool in pan on rack 10 minutes. Invert bundts onto rack and cool completely.
For the Chocolate Glaze:
Melt over low heat, stir in cream. Mix together.
Transfer to a piping bag.
For the Glaze:
Mix together confectioners sugar and milk.
For the Assembly:
Drizzle both glazes onto cooled mini bundts.
Video
Notes
Measure your flour correctly! Adding too much flour to the recipe is the most common mistake. The best, and easiest way to measure flour is by using a scale. If you don't have one then fluff your flour with a spoon, sprinkle it into your measuring cup, and use a knife to level it off.
Make sure to butter and flour your pan well. Non stick pans will still stick! You can use baking spray if you're in a hurry too.
When you make the glaze add the milk in a bit at a time and give it a good mix. It has a tendency to go from giant clump to too watery in a flash.
These cakes will keep for about a week in the refrigerator if stored in a sealed container. Allow to come to room temperature before serving.