Would the recipe for Old Fasioned Chocolate Cake work if I substitute the chocolate with white chocolate?
Nigella's Old Fashioned Chocolate Cake (from FEAST) is a chocolate sponge cake coated in a ganache-style chocolate frosting. The cake itself is made with cocoa powder so would still need to be made as a dark chocolate cake, as you would not be able to substitute the cocoa powder with white chocolate. White chocolate is mostly made up of sugar and fat (cocoa butter), with some milk solids, vanilla and lecithin. If you used this instead of the cocoa powder then the cake would be heavy and probably slightly greasy.
For the frosting it is possible to make a white chocolate ganache, but the ratios of chocolate to cream are different as white chocolate has no cocoa solids (and the cocoa solids will help the frosting to set). So you would not be able to simply swap dark (bittersweet) chocolate for white chocolate in the recipe. You would also need to adjust the amount of butter and sugar in the frosting to compensate for the extra fat and sugar in the white chocolate. So unfortunately we are unable to advise on the ratios of the various ingredients. However, if you are a lover of white chocoate we would suggest trying Nigella's White Chocolate Cheesecake.