Preheat oven to 350F. Grease a 7-inch loaf pan with cooking spray or butter.
In a large bowl, whisk the eggs and sugar until combined. Whisk in the oil, vanilla and milk.
Now add all purpose flour, baking powder and salt. Mix until fully incorporated ( do not over mix the batter).
Separate ½ cup of batter in another bowl. Add unsweetened cocoa powder to half of the batter and mix until combined.
Alternately, spoon the vanilla and chocolate batters into the prepared loaf pan. Swirl it through the batter with a toothpick to create a marble effect.
Bake for 40-45 minutes or until the toothpick inserted comes out clean. Let it cool completely, then slice to see the beautiful marble effect.