It may not be Poilane or Mulot, but we stayed in the Marais last week and were referred to this bakery by the apartment agency's neighborhood guidebook. Everything we bought there was delicious, including their regular baguette, which was more rustic than supermarket loaves; it stayed fresh for 2 days. We had a small (5") pear/cherry cake that was not very sweet, a rectangle of classic French flan, an apple tart, and some other treats to nibble on, and they were all very good and reasonably priced.
I'm generally amazed by how these tiny local bakeries provide well over 100 different items every day. from rustic breads to gorgeous decorated patisserie, huge quiches, viennoiserie, sandwiches, etc. Are there 30 people in the basement?