Melt the chocolate chips in the microwave in 20 - 30 second intervals. Stir and repeat the chocolate is melted.
Combine the cake mix, eggs, vanilla extract, sour cream, vegetable oil, sugar and water in a mixing bowl using an electric mixer or stand mixer. Beat on high for 5 minutes or until everything is combined.
Remove ⅓ of the cake batter and stir it into the melted chocolate mixing well.
Spread half of the yellow cake mix batter into the cake pan that has been sprayed well with cooking spray. Then spread the chocolate batter on top followed by the remaining yellow cake batter. Using a butter knife, swirl the batters together to create the "marbled" look.
Bake for 30 to 35 minutes in a preheated 350 degree oven.
Let the cake cool in the pan for 10 to 15 minutes.
Remove the cake from the pan and let cool completely.
Sprinkle with powdered sugar before serving