Just cooking one of our butternuts today:), but no seeds inside?, any reason why not?, could it be i used pumpkins or courgettes to fertilise them as the was no male flowers on the butternuts around at the time.
(http://i1191.photobucket.com/albums/z480/notts_phil/IMAG00481.jpg)
Never seen that before... You may have answered you own question with the polination...
I get this frequently. Butternuts are of the 'cucurbita moschata' species of squash, and they are the most warmth loving of the squashes. Quite often it is too cool here for proper pollination. But the plants have a tendency to 'think' they have been pollinated, when they haven't and as a result they will still grow fruit.
It is too soon. I have several which have grown well and I have harvested those which are at the right size will keep for a bit (no seeds) but as yet the rest of the crop are still developing
They are not a natural crop for the UK which may be the reason why they are not acting as we expect.
The fruit should ripen at the end of this month so small squash will produce seeds for next year.
Just harvest what you get and eat them.