Start by preheating your oven to 325°F (163°C). Position a rack in the center of the oven
In a small bowl, pour the bourbon and scrape the seeds from the vanilla bean into the bowl. Stir to blend well.
In a medium-sized bowl, whisk together the flour, cornstarch, baking powder, baking soda, and salt. Set this dry mixture aside for later.
Using an electric mixer, beat the softened butter, sour cream, and sugar in a large bowl until light and fluffy, about 2-3 minutes. This step is essential for creating a light cake texture.
Add the eggs and egg yolk to the butter mixture, one at a time. Beat well after each addition. Then, beat in the bourbon-vanilla mixture until fully incorporated.
Add the dry mixture in two additions, alternating with the milk (add milk in one go), beating just until blended after each addition. Be careful not to overmix to maintain the cake’s tender texture.
Divide the batter into two equal portions. To one portion, add the cocoa powder and milk, and stir until fully incorporated, creating a chocolate batter.
In the prepared mini Bundt pan, spoon in a layer of vanilla batter. Follow with a layer of chocolate batter, and top it off with the final layer of vanilla batter. Use a skewer or knife to gently swirl the two batters together, creating a marbled effe
Place the pan in the oven and bake for 20-25 minutes, or until a tester inserted into the center of a cake comes out clean. Keep an eye on the cakes to avoid overbaking.
Once baked, remove the mini Bundts from the oven and allow them to cool in the pan for about 10 minutes. Then, invert the pan onto a wire rack and cool the cakes completely.