What is Reynaud's opinion for Vianne's cooking chocolate? How is Vianne interacting with Reynaud through cooking chocolate ?
First of all, it is important to note that if Vianne is the protagonist of this novel, Reynaud is certainly the antagonist. Reynaud is the town priest (in the middle of Lent no less). Vianne represents everything Reynaud stands against.
On a weekly basis he preaches selflessness and purification through fasting, sacrificing, suffering. He preaches against pleasure and indulgence. He looks down on members of his parrish who struggle and are unable to deny themselves life's pleasures - he considers them physically and spiritually weak. Through the chapters in which he is the narrator - we also see that it gives him a certain satisfaction to make comparisons of himself to these weaker people - it makes him feel spiritually superior.
Therefore, while Vianne comes across as Reynaud's biggest enemy, and while he does everything in his power to bring her, her chocolaterie, and her plans for the chocolate festival (at the end of Lent - and the end of the story) down - secretly, it is obvious that Reynaud uses Vianne to ease his own secret guilt and boost his spiritual ego.
In the end, it turns out that she is simply too much for him. Her interactions with him are never malicious, never competitive, and generally unengaged. She does not react to him - which enrages him. When he is found in the store window the morning of the chocolate festival - asleep and in a chocolate stupor - it seems as if he has finally given in to the old saying, "If you can't beat them, join them."