Preheat the oven to 325 F (160 C). Butter a loaf pan or line with parchment paper.
Cream together the butter and sugar until very light and fluffy. Make sure to scrape down the sides a few times.
Add the eggs one at a time, letting the previous one become fully incorporated before adding the next. Make sure they are room temperature, so that your batter doesn't split!
Add the vanilla extract to the eggs, butter, and sugar. Mix together.
Measure out the milk in a bowl/cup and mix the flour, baking powder, and salt together in a separate bowl.
Sift in half of the dry ingredients and mix together until just incorporated. Then add half of your room temperature milk and mix until incorporated. Sift in the remaining dry ingredients, mix, then add the remaining milk. Once incorporated, give the mixture a few folds with a spatula to make sure everything from the bottom has been mixed.
Now, remove about 1/3 of the vanilla batter and add to a separate bowl. Add the cocoa powder and mix well until it becomes chocolate batter!
Add half of the vanilla batter to the loaf tin, pour the chocolate batter into the middle, then add the rest of the vanilla batter on top.
Using a skewer, knife, or spatula, make a zigzag pattern through the batter (this will create the marble effect).
Tap the pan against the counter a few times (this will ensure that there aren't any air bubbles in the cake). Then, using the skewer/knife/spatula, slice straight through the middle of the cake. This will give the nice crack on top that is characteristic of a pound cake!
Bake for 50-55 minutes, or until a cake tester/skewer comes out clean. Enjoy!