Preheat your oven to 175°C (350°F) and prepare a loaf pan by lining it with a baking sheet or greasing it to prevent sticking.
In a medium bowl, combine flour, salt, and baking powder, then set aside.
In a larger bowl, mix eggs, sugar, and olive oil until it turns creamy and slightly fluffy, for about 2-4 minutes.
Gradually add the dry ingredients to the wet mixture, along with the milk (1/2 cup). Mix until well combined, avoiding overmixing.
Take a cup of the batter and add to a small bowl. Then mix in cocoa powder and milk (1 tbsp) until it becomes chocolatey.
Pour the vanilla batter into the pan (split in two - three) and add the chocolate batter in dollops, and keep repeating the process. Use a skewer to create a marbled swirl effect.
Bake for 35-45 minutes, and use a skewer to check for doneness. If it comes out dry, the cake is ready.
Allow the cake to cool completely before cutting to maintain its texture. Enjoy your marble cake!