Preheat your oven to 350 degrees F. I always stress the importance of an oven thermometer! If you're not using one, pick one up and I promise your baking will change for the better!
Prepare your bundt pan by rubbing it down with some unsalted butter. Make sure to really get into all the little spots! The butter is what will grab the flour and keep your cake from sticking. A bundt cake that sticks to the pan is a hot mess that no one wants to deal with!
Add a bit (no more than 1/4 cup) of flour to the bundt pan and rotate the pan around over your sink while making sure you get flour over the whole inside of the pan. I do this process over the sink because it is a bit messy.
In the bowl of a stand mixer or with a hand mixer, beat your unsalted butter, sugar, and salt until light and fluffy. This will take a few minutes.
In a medium bowl, mix together your flour and baking powder. Set aside.
With the mixer on low-medium, add your eggs one at a time. Give each egg a few seconds to incorporate before moving on to the next one.
Once all of your eggs have been added, scrape down your bowl and paddle. There will be some chunks of the butter/sugar mixture that didn't incorporate well.
Turn your mixer back on for a few seconds to make sure everything is incorporated.
Add your milk and vanilla. Mix until incorporated.
With your mixer on low, slowly add the flour and baking powder.
Mix just until it's all starting to come together. Turn your mixer off, scrape down your paddle and bowl and fold any ingredients together until fully incorporated. This is a pretty thick batter. Don't worry, it's suppose to be that way.
Take half of your batter and place it into the bowl you had your flour and baking powder in. This is just my way of not having so many dishes to clean afterward 😉
Add your cocoa powder to the batter that you left in your mixing bowl.
Mix on low until mostly incorporated.
Take your vanilla batter and scoop it into your prepared bundt pan. I scoop parts of it with my spatula so I can get a somewhat even amount into the pan all the way around. It doesn't have to be perfect!
Now take your chocolate batter and fold it just to make sure there's not any lumps of cocoa powder left around the edges.
Once it's all incorporated, take scoops of the chocolate batter and place them on top of the vanilla batter in the bundt pan. Again, this doesn't have to be perfect at all. Once all the batter is in the pan, take your spatula and even out the top just a bit. You just want to make sure your batter isn't lopsided in the pan so it bakes evenly.
I also tap my pan on the counter a few (2-3 times) to help even the batter out.
Place the bundt cake into your oven and set your timer for 24-28 minutes.
At 28 minutes, lightly touch the top of your bundt cake. If it springs back, it's ready! If it's not, add a few (2-4) minutes to the baking time.
When it's done, remove it from the oven and let it set in the pan for an additional 5 minutes.
After 5 minutes, remove it from the pan by placing your cooling rack over the pan and flipping it out.
If you've prepared your pan correctly, the bundt cake should slide right out and have the shape of the pan on it!
Let the bundt cake cool completely before adding the icing!