Dr John Sentamu witnessed the production of the first Fairtrade version of the chocolate biscuit bar at Nestlé's factory in York.
The launch of the Fairtrade four finger bar follows calls from the Archbishop, members of the public and groups like Stop the Traffik for the ethical production of chocolate.
"It goes to show people can make a difference," he said. "The fact that Nestle have listened to local campaigners and invested considerable time, effort and financial support to make this Fairtrade biscuit a reality is fantastic news."
Speaking of workers he visited on a cocoa co-operative in the Ivory Coast, he said the Fairtrade Kit Kat was a "step forward in giving them the justice, recognition and pay they deserve".
He said the next step for the confectioner was to make all its other products Fairtrade, including his personal favourite, the Yorkie bar.
"This will depend largely on the Ivory Coast producing the large quantity and quality needed for Nestle to deliver the product," he said.
"Nestle must be congratulated in going beyond the profit margin.
"In the Ivory Coast, Nestle has invested heavily in a research centre for delivering millions of high quality cocoa trees, which also has the potential to develop millet, sorghum, cassava and coffee.
"This is breath-taking for all concerned. Fairtrade Yorkie – here we come!"
Steve Chalke, founder of Stop The Traffik and the United Nations Special Adviser on Community Action Against Human Trafficking welcomed the Fairtrade Kit Kat but urged Nestlé to work towards making all its products Fairtrade.
"We are relieved for the cocoa farmers and children in Ivory Coast," he said. "The surrender of Nestlé demonstrates that by making a simple consumer choice ordinary people can hold multi-nationals to account.
"Though we understand that it is hard to make all products ethical overnight, we want to see that this is more than a token gesture.
"So, we intend to keep the pressure on Nestlé until their commitment is global and product wide, like their competitor Mars.
"No chocolate should have the bitter aftertaste of slavery. Therefore our campaign continues."