Whisk the milk, sour cream and rum together in a liquid measure and let come to room temperature.
Preheat the oven to 350 degrees F and make sure an oven rack is placed in the middle of the oven.
In a large bowl or stand mixer whip the butter on medium speed for 1 minute. Add the granulated sugar and beat for 4-5 minutes more. Mix in the vanilla.
Add the eggs and egg yolks, one at a time, mixing just until combined in between additions. Scrape down the sides of the bowl.
Combine the cake flour, baking powder, baking soda and salt.
Add 1/3 of the dry ingredients to the batter and mix just until combined. Add 1/2 of the milk/sour cream mixture and mix.
Add another 1/3 of the dry ingredients, mixing just until combined. Add the last 1/2 of the milk/sour cream mixture and beat until just combined.
Add the final 1/3 of the dry ingredients and mix just until combined, scraping down the sides of the bowl.
Grease and flour bundt pan. I use shortening, and make sure every crevice of the pan is coated! Pour batter evenly into the pan and bake at 350 degrees F for about 50 minutes or until a toothpick inserted in the center comes out clean, or with few crumbs.
While cake is baking, make the glaze sauce. Add sauce ingredients to a saucepan over medium heat. Stir well until butter has melted and mixture is smooth. Bring mixture to a boil, stirring, and boil for 2 minutes.
Allow the cake to cool for in the pan for at least 15 minutes before gently inverting it. (I like to remove the cake first, to make sure to get it out easily). Then wipe the pan clean and put the cake back in it. Poke holes all over the cake and gently pour most of the sauce over the cake, reserving just a little.
Allow the cake to soak up the glaze and then invert it onto a plate or serving platter. Spoon remaining glaze on top.
Store rum cake well-covered in the refrigerator.
Storing: Rum cake can be stored in the refrigerator for up to 5 days. Freezing: Bake the cake but do not make the glaze sauce. Allow the cake to cool completely and store covered well in the freezer for up to 3 months. Thaw the cake completely by allowing it to sit in the refrigerator overnight. When ready to eat, make the sauce, poke holes in the cake and gently pour the glaze on top. Allow it to rest for at least 30 minutes before serving.