There are two great places in Paris that ARE NOT closed on Sundays which is quite an exception :). They are both in St Germain, just steps from each other, and they are both world-famous.
The first is Gérard Mulot (76 Rue de Seine, 75006), a boulangerie and “trateur” where you can take away everything from excellent bread, sandwiches, prepared dishes to cakes and chocolates. It was founded 25 years ago by Mr and Mrs Mulot and still belongs to the family. It is also one of those rare places which takes ONLY cash.
The second is, of course, the shop of the renown French pâtissier Pierre Herme (72 Rue Bonaparte, 75006) where the desserts are almost like objects of art. It was Pierre Herme who was the first to introduce such macaroons flavours as “olive oil” or “truffle” and he is the author of my favourite dessert “Ispahan” (rose flavour macaroons biscuits, rose cream, fresh raspberries and lychee). There is another Herme shop in 15th arrondissement and there used to be one in 8th which was closed 2 years ago or so. I remember going there almost every day as it was very close to where I live :).