I have a question about the recipe "Apple and Olive Oil Cake with Maple Icing" from Yotam Ottolenghi. Where an I find the muscovado sugar
Whole Foods should have it
Most upscale markets will carry it, but if you can't find it you can substitute brown sugar.
I order it from Amazon, Billington's brand, 10 pounds at a time - much less expensive than buying it at WF. You can substitute regular grocery store brown sugar of course, but muscovado sugar (being a true partially-refined brown sugar, not white sugar with molasses added back in) has a deeper, more complex flavor.
King Arthur also has it in their catalog.