Preheat oven to 350F/180C. Line a baking pan 9 x 5 x 3 inch with parchment paper.
In a bowl, whisk together flour, salt, baking powder, and baking soda. Keep aside.
In a different bowl add the rest of ingredients (except cocoa powder) and whisk until well combined.
Add the flour and mix gently.
Transfer ¾ of the batter to another bowl and sift in the cocoa powder. Whisk until well combined but do not overmix.
Take ¼ cup from the plain batter and pour in the middle of the previously prepared pan.
Pour ¼ cup from the cocoa batter in the center of the plain batter that’s in the pan.
Pour ¼ cup from the plain batter in the center of the cocoa batter you’ve just poured in the pan, keep repeating this step alternating between batters.
Place the pan on the middle rack and bake for 50 minutes or until a toothpick inserted in the middle comes out clean.
Keep the cake in the pan for 5 minutes, then transfer to a cooling rack.