The original Finnish recipe for Molotov cocktail (ethanol, tar and gasoline mixture) was effective against Russian tanks during the Winter War (before WWII started). T-28 was the biggest Russian tank during the war. The original intent of the cocktail was to blind the vision ports of the tank. It was quickly discovered that if the cocktail was thrown close to the engine air intake, it would suck the the flames inside and the engine would catch a fire. The Red Army lost 1 919 tanks in the winter war battles, of which 436 due to a fire (most of the fires were caused by Molotov cocktail).

In the WWII (continuation war for the Finns) all parties had better weapons and usually better tanks and tank tactics. I have no information of the use of Molotov cocktails against tanks in any scale during the WWII.

source: Mannerheim-linja — talvisodan legenda, Antero Uitto ja Carl-Fredrik Geust, 2006. ISBN 951-20-7042-1.