it is for Authentic Baguettes at Home From America's Test Kitchen Season 15: Baguettes at Home
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The recipe at the ATK website requires registration, but I found it elsewhere:
The recipes says the the malt powder is optional, so if you don't have it, you don't need to find a substitute.
Cynthia is a trusted source on Bread/Baking.
You can use a small- very small - as a substitute. If you're having trouble finding barley malt powder, home brew stores are an excellent and inexpensive source.