Line a loaf tin with parchment paper.
Cook the butternut squash, steam it if you can or boil until soft and strain. Add it to a food processor.
Melt the chocolate and butter in the microwave or in a bowl over a pot of simmering water.
Blend the melted chocolate and butter with the butternut squash until silky smooth.
Pour the mixture into the loaf tin, smooth with a spoon or spatula, and gently tap to let any trapped air out.
Pop in the fridge to set for at least 3 hours.
Slice and serve. To get perfect slices, dip a sharp knife in boiling water, dry with a kitchen towel, and slice. Clean and repeat for every slice.