Step 1: Prepare Your Pan and Oven (Mise en Place)
First, position a rack in the center of your oven and preheat it to 350°F (175°C). Next, meticulously prepare a 9×5 inch loaf pan. This is a crucial step to prevent the cake from sticking. Grease the pan thoroughly with butter or non-stick spray. Then, line the pan with a piece of parchment paper so that it covers the bottom and has a 2-inch overhang on the two long sides. This parchment “sling” will allow you to easily lift the finished cake out of the pan. Grease the parchment paper as well.

Step 2: Combine the Dry Ingredients
In a medium bowl, whisk together the 1 ¾ cups of all-purpose flour, 2 teaspoons of baking powder, and ½ teaspoon of salt. Whisking them together ensures the leavening and salt are evenly distributed, which is key for a uniform rise and flavor. Set this bowl aside.

Step 3: The Creaming Method
In the large bowl of a stand mixer fitted with the paddle attachment, or in a large bowl with a hand mixer, place the ½ cup of softened unsalted butter and the 1 cup of granulated sugar. Beat on medium-high speed for a full 3-5 minutes. The mixture should transform from a grainy, yellow paste into a very pale, light, and fluffy cream. This step is vital; it’s where you are whipping air into the fat, which creates the foundation for a tender, finely-crumbed cake. Scrape down the sides and bottom of the bowl a couple of times during this process.

Step 4: Add Eggs and Vanilla
Add the 2 large, room-temperature eggs, one at a time, beating well for about 30 seconds after each addition until it is fully incorporated. After the last egg is mixed in, scrape down the bowl again and add the 2 teaspoons of pure vanilla extract. Beat for another minute until everything is well combined.

Step 5: Alternate Wet and Dry Ingredients
In a small bowl, whisk together the ½ cup of room-temperature sour cream and the ¼ cup of room-temperature whole milk until smooth. Now, you will add the dry mixture and the milk mixture to the butter base in alternating additions. This gradual process prevents you from overwhelming the batter and keeps the emulsion stable.

• With the mixer on low speed, add about one-third of the dry ingredients and mix until just combined.

• Add half of the sour cream/milk mixture and mix until just combined.

• Add another third of the dry ingredients.

• Add the remaining sour cream/milk mixture.

• Finally, add the last of the dry ingredients and mix only until the last streaks of flour disappear. Do not overmix!

Step 6: Divide and Create the Chocolate Batter
Transfer approximately half of the vanilla batter to a separate medium bowl.
In a small bowl, whisk together the ¼ cup of cocoa powder and the 2 tablespoons of hot coffee or water until a smooth, thick paste forms. Add this chocolate paste to one of the bowls of batter and use a spatula to gently fold it in until it is uniformly combined and no streaks remain. You now have one bowl of vanilla batter and one bowl of chocolate batter.

Step 7: The Marbling Technique
Drop alternating spoonfuls of the vanilla and chocolate batters into your prepared loaf pan. There’s no need to be perfectly neat here; you can drop them checkerboard-style or in random blobs. Continue until you’ve used all the batter, creating a couple of layers.
Now, for the swirl. Take a butter knife or a wooden skewer and insert it into the batter. Make 2-3 long, sweeping “S” shaped cuts through the length of the pan. Then, make one or two cuts through the width of the pan. That’s it. The most common mistake is over-swirling, which will result in a muddy, uniformly brown cake rather than a distinct marble pattern. Less is more.

Step 8: Bake and Cool
Place the loaf pan in the preheated oven and bake for 50-60 minutes. The cake is done when it is golden brown, has pulled away slightly from the sides of the pan, and a wooden skewer or toothpick inserted into the center comes out clean or with a few moist crumbs attached.
Let the cake cool in the pan on a wire rack for 15-20 minutes. Then, use the parchment paper sling to carefully lift the cake out of the pan. Let it cool completely on the wire rack before slicing. This final cooling period is important for the cake to set properly.