Preheat oven to 350oF (180oC).
Line cupcake pan with cupcake liners.
In a bowl, add sugar. Then, sift in flour, baking powder and salt. Mix to combine.
To the same bowl, add oil, water and vanilla extract. Whisk until dry flour no longer visible. Do not over mix.
Transfer half of the batter to another bowl.
Add cocoa powder to one of the bowls.
Add about 1/2 tablespoon of vanilla batter to cupcake liner. Then, top with 1/2 tablespoon of chocolate batter and then vanilla batter again. Alternate between vanilla and chocolate batter until all batter is used.
Use a toothpick to lightly swirl the vanilla and chocolate batter together on the top of the cupcakes.
Bake in preheated oven 350oF (180oC) for 20 minutes, or until toothpick comes out clean.
Let cupcakes cool down in pan for about 10 minutes. Then, transfer on wire rack to cool completely.