Bake a baguette in the usual manner.
Insert a plastic rod from one end to the other, nearly perforating the far end. The rod should have a soft cap that slides over the end of the baguette, and can be sealed around the breadcrust with food-grade glue, such as that which is used to manufacture soda
The rod should be coaxial, , and should be designed so that it has a suction component (presumably the internal portion) and a throughput component (presumably the external mantle, if you will). The rod would be necessarily perforated. I am imagining the rod as having perforations leading from the internal core to the outside, so that the internal and external sections are equally connected to the environment of the bread.
The two axes of the plastic rod will be connected to a small air pump (for suction) and a tank of hydrogen (for input).
When both are activated at equivalent flow rates, the carbon dioxide and air within the bread will be sucked out through the rod, and hydrogen will replace it.
Obviously, some hydrogen will be wasted.
Once the bread is presumed to be filled entirely with hydrogen gas, the rod is removed and the hole is quickly plugged with another soft cap, which is glued on.
If done correctly, the bread should be lighter than its standard compatriots. It might possibly float, if you are lucky.
And, the best part: break bread near a candle, and have instant toast.
Inspired by the idea "The Bread Zeppelin" by [acurafan07] (see link)