I need help from you bakers!

I have a recipe for cupcakes (I guess they are vanilla?). I'd like to add cocoa to make them chocolate, but I'm not sure what I need to remove/add to make it work, or even how much cocoa I should use. Here's the recipe:

1 cup milk
2 tbsp vegetable oil
1 large egg
1 tsp vanilla extract
1 1/4 cups white flour
1/2 cup sugar
2 tsp baking powder
1/4 tsp salt

Makes 6 cupcakes.