To make sure both chocolate and flour batter don’t dissolve, keep the consistency sufficiently thick.
Layer both chocolate and vanilla batter carefully instead of pouring in all at once.
Drop chocolate into cake batter with a tablespoon throughout the batter and not in only one place.
Instead of a toothpick, use the pointed edge of the knife to make better swirl patterns.
Use your creativity to create desired patterns and make your own version.