Put the ancho pepper in a small saucepan and cover with water. Bring to a boil. Reduce heat and simmer until pepper is soft, about 10 minutes.
Remove the pepper from the liquid. When cool enough to handle, stem the pepper, split it open, and scrape out the seeds. Meanwhile, put the squash in a saucepan and cover with water.
Bring to a boil over medium-high heat. Reduce heat and simmer until the squash is tender, about 10 minutes. Drain well and transfer to a food processor. A blender is not recommended.
Add the pepper and the cumin and process until smooth, stopping to scrape down the sides of the bowl. Season with salt and cayenne. Divide among 4 plates and serve immediately.