Siman 258
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א מֻתָּר לְהַנִּיחַ מִבְּעוֹד יוֹם כְּלִי שֶׁיֵּשׁ בּוֹ דָּבָר קַר עַל גַּבֵּי קְדֵרָה חַמָּה, שֶׁאֵין זֶה כְּטוֹמֵן בְּדָבָר הַמּוֹסִיף הֶבֶל.
1 It is permissible on Erev Shabbos while it is still day to put a vessel with cold food on top of a warm pot because this is not insulating with something that gives more heat.