Buttercup is a dark green winter squash (Cucurbita maxima - not to be confused with the butternuts which are Cucurbita moschata). After trying many different squashes over a few years I found Buttercup to be the open pollinated variety with the best flavour. They have a sweet dry nutty flesh which can be used in a variety of recipes. As the flesh is quite dry they hold their shape when used in curries and stews. They can be grated raw into salads too.
Later I read that they were bred at North Dakota State University as a squash to substitute sweet potatoes. I have been growing only this variety for a few years and saved the seed from squash which stored for a few months. They are varied in shape from flat round to the more squashed and pear shaped form. There is often light green/grey ‘button’ at the blossom end of the fruit.