Special equipment: Witch-shaped cookie cutter
Preheat oven to 400 degrees F.
Make the Witch Croutons: Trim and discard crusts from bread. Spread 1 side of each slice evenly with butter and season with salt and pepper. Cut into witch shapes with cookie cutter and arrange, buttered side-up, on a baking sheet. Toast bread, turning once, in middle of oven until golden brown, about 4 minutes. Transfer the croutons to a rack to cool.
Make the Soup: Put squash, cut side-down, in a greased shallow baking pan and roast in the oven until very tender, about 1 1/4 hours. Cool and scoop out flesh.
Puree squash in batches in a blender with jalapeno, cumin, and broth until very smooth (add water, as needed, to facilitate pureeing). Transfer to a 6 to 8-quart heavy pot and stir in the 5 cups of water. Simmer, stirring, for 10 minutes, then stir in lemon juice, enough water to thin to desired consistency, and salt and pepper, to taste.
Divide soup among bowls, top with a witch crouton, and drizzle drops of hot sauce around the sides.
Toasts may be made 2 days ahead, cooled completely, and kept between layers of paper towels or waxed paper in an airtight container, at room temperature. The squash puree may be made 2 days ahead. Cool completely; then chill, covered.