Preheat oven to 350F. Grease and flour your loaf pan. Line it with parchment paper.
In a large bowl, mix together, all purpose flour, salt, baking powder & baking soda. Keep it aside.
Melt the chocolate in a small bowl. You can melt it on the stovetop using the double boiler method or melt the chocolate in the microwave in 30-second increments, stirring between until fully melted. Make sure there are no lumps. Set aside.
To another bowl, add eggs and sugar. Beat until well combined. Add oil, and vanilla extract. Beat again until all ingredients are mixed well.
Add the dry ingredients and milk alternatively, little by little and mix well using a spatula. Once the vanilla batter is ready, transfer half the batter to the bowl of melted chocolate.
Mix gently until combined. Now both vanilla and chocolate batters are ready.
Now it's time to add the batter to the prepared loaf pan. First, add the vanilla batter and add the chocolate batter on top of it.
Bake for 45-50 minutes or until fully cooked. To check if your cake is done, insert a toothpick in the middle of the cake. Once the toothpick comes out completely clean, it’s time to take the cake out of the oven.
Let the cake cool for a few minutes. Transfer to a cooling rack, and let the marble cake come to room temperature. Cut and serve.