Is there something else I can use for butter, like coconut oil, for the Rosemary Remembrance Cake, or will that ruin the taste completely?
Nigella's Rosemary Remembrance Cake (from FEAST) is a madeira style cake that is flavoured with apple and rosemary. Although the rosemary adds its slightly resinous perfume to the cake, the flavour and richness of a madeira cake comes from butter and eggs.
Unfortunately using something like coconut oil could cause problems. First coconut oil doesn't add any flavour and the cake could end up tasting rather flat. Second coconut oil is 100% oil, whereas butter is ony 80% butterfat (the rest is milk solids and water). Using coconut oil could mean that the cake becomes slightly greasy. So on balance we would prefer to use butter if at all possible.