Preheat oven to 350 degrees. Grease an 8 or 9 in loaf pan.
In a bowl, whisk together flour, baking powder, baking soda and salt. Set aside.
In a separate smaller bowl, add in 1/3 cup sugar, cocoa powder and water. Whisk together until smooth. Set aside.
In a mixing bowl, beat together remaining 1 cup sugar and butter. Add in egg, then, egg white. Then, add in vanilla and yogurt. Mix until combined.
Gradually add in flour mixture and mix until combined.
Take one cup of batter and add to cocoa mixture. Combine well.
Layer batter in loaf pan, starting with vanilla mixture, then, chocolate. Swirl with butter knife for a marble effect.
Bake for 45-50 minutes or until toothpick inserted comes out clean.
Let cake sit for 10-15 minutes in before removing from pan.