Maou no Amai Keiyaku(2007)

Maou no Amai Keiyaku

Despite being the grandson of "witch", Ryou has no magical abilities. But he's content making delicious pastries at the "unbreakable" cafe run by his grandmother. But when she sends him to the cafe's mysterious warehouse, he meets... a demon king?! And what's this about having to make a binding contract with him?