Anyway, since you grow in hardiness zone 4b, you should finish it on the windowsill as soon as you have to fear frost. If this picture is fresh, this fruit is pollinated in August.

So winter squash can continue to ripen even off the vine?

Yes, I took the picture yesterday, along with these. All were planted June 18.

My ripest-looking Buttercup:

Red Kuri:

Sweet Dumpling:

I have successfully ripened winter squash off the vine. In my experience, it is best if it has some darker color to it. But some creep (I garden at a community garden) pulled one of my green butternuts off the vine last week and so far it hasn't shriveled. I have it under a plant light. Usually, if it is too young to make it, it will shrivel pretty fast. Your BN does look like a Waltham which I grew this year. Last year, I put a mostly green Butternut Bush squash under the lamp for two months and it was so sweet! Hopefully, someone who has grown your variety can comment more on it.

Thanks, Gardengal! We used to start seeds on a 4-tier plant light. I'll have to see if any of the bulbs are still functional in case they're needed.

You're welcome, Alisande! Happy gardening :)