Cut the vanilla bean in half and use your knife to scrape out the seeds. Add the seeds and the pod to a medium saucepan. Add the rest of the ingredients. Bring to a boil over medium heat, then reduce heat slightly and boil for about 5 minutes, stirring constantly, until sauce is slightly thickened. Allow to cool before storing in an airtight container in the refrigerator. Reheat for a minute in the microwave before using.
Make the cake:
Preheat oven to 350 degrees. Butter and flour two 8-inch cake pans and line the bottoms with parchment. Set aside.
In the bowl of a stand mixer fitted with a paddle attachment, beat together the butter and brown sugar until fluffy. Add in the eggs, one at a time, mixing thoroughly after each addition.
In a bowl, whisk together the flour, baking powder, baking soda and salt. In a measuring cup, combine the buttermilk and vanilla.
Slowly add 1/4 of the flour to the butter-sugar mixture, then 1/3 of the milk. Continue until all of the buttermilk and flour is added, ending with the flour. Mix just until combined.
Divide batter into the prepared cake pans. Bake for 20-25 minutes, or until a toothpick inserted into the center of the cakes comes out clean. Allow to cool for 20 minutes in the pans, then turn out onto a cooling rack and allow to finish cooling completely.
Make the apple filling/topping:
In a saucepan, whisk together the corn starch, brown sugar and milk until smooth. Add the butter. Cook over medium heat until thick and bubbly. Add the apples and cook over medium-low heat just until the apples soften.
Allow apple filling to cool slightly before using. Syrup will thicken slightly as it cools.
Assemble the cake:
Place one of the cake layers on a cake plate (one with a lip does best for this messy cake). Spoon about half of the apples over the cake, spreading evenly. Place the second layer on top, and top with the remaining apples, spooning them more onto the center of the cake. Top with as much of the syrup as you like.
Serve cake with the Easy Vanilla Bean Caramel Sauce and lightly whipped cream.