Preheat the oven to 350° F. Grease and set aside 9x5 inch loaf pan.
In the bowl of a stand mixer fitted with the paddle attachment, beat together butter and sugar until light and fluffy, 3-5 minutes.
Add vanilla. Add eggs one at a time, beating after each addition. Mix in greek yogurt.
Once fully combined, add salt, baking powder, and half the flour. Mix until incorporated.
Add the milk and stir. Once incorporated, add the rest of the flour.
Remove half of the batter from the bowl and set aside. This will be the vanilla cake.
Add the cocoa powder to the remaining batter and mix until fully combined.
Add the batter to the prepared loaf pan. Alternate scoops of chocolate and vanilla batter. Once all batter is in the pan, use a butter knife to swirl the flavors together.
Bake at 350°F for around 1 hours and 20 minutes, or until a tester inserted in the center of the cake comes out clean. Let cool fully before slicing. Enjoy!