Line an 9-inch springform pan with parchment or lightly spray with cooking spray.
In a small bowl, combine the oreo crumbs and coconut oil. Press into the bottom of the pan in an even layer.
Fill a pot fitted with a steamer basket with about 2” of water. Bring to a boil, add the squash to the basket and cover, steaming until tender, about 15 minutes.
When the squash is done, add it along with the chocolate to a blender or food processor and cover for 5 minutes, until chocolate has melted. Blend until very smooth.
Pour the squash/chocolate mixture over the oreo crust. Gently tap the pan to distribute the mixture evenly and smooth the top with a spatula.
Chill the cake overnight.
Slice and serve with a dusting of cocoa powder and a handful of berries, if desired.