Hi Kenny! Sounds like you have a fun evening planned! There are several options for traveling from Hollywood Studios to Epcot. The bus is the fastest; the ride takes about 15 minutes, although sometimes you have to wait at least 15 minutes for the bus to pick you up. You can also walk from Hollywood Studios to Epcot in about 30 minutes, or you can take the boat, but it takes even longer, about 40 minutes. Although walking or the boat would take longer, these routes would allow you to enter Epcot at the International Gateway, which would be closer to your reservation at Chefs de France. For this reason, I think if you have the energy to walk to Epcot, you would reach Chefs de France the quickest by walking. However, if the bus happens to be waiting when you leave Hollywood Studios or you're too tired to walk between the parks, hop on the bus. (It will drop you off at the main entrance to Epcot, so you'll still have to do some walking to reach Chefs de France.)
I recommend that you sit near one of the exits at Fantasmic so you'll be able to leave the theater quickly. The show takes about 30 minutes, and if you're sitting near the front or far left (facing the stage) of the theater, it can take you another 30 minutes to exit, which would make you late to your Chefs de France reservation. Position yourself near an exit so you'll be at the front of the crowd leaving Fantasmic.
Hope you have a magical time!