Remove pan from oven and run a knife through the batter to marbleize.
From the dallasnews.com
With a long thin knife, gently swirl the batters to marbleize them.
From the dailynews.com
Swirl chocolates together with a knife to marbleize.
From the news-journalonline.com
Draw a knife through and swirl to marbleize.
From the courier-journal.com
More examples
Make something look like marble; "marbleize the fireplace"
Marbleizing or faux marbling is the preparation and finishing of a surface to imitate the appearance of polished marble. It is typically used in buildings where the cost or weight of genuine marble would be prohibitive. ...
(Marbleizing) A faux finish that creates the illusion of marble.
To give concrete surfaces the look and gloss of marble, through a combination of color layering and finishing techniques.
To paint wood or other material to the have the appearance of marble.