Preheat oven to 375 degrees F. Lightly coat two 15x10x1-inch baking pans or shallow roasting pans with cooking spray. Divide squash evenly between two large bowls. Set aside.
In a small bowl combine half of the brown sugar, the cocoa powder, and 1/2 teaspoon of the salt. Stir in 2 tablespoons of the melted butter. Pour cocoa mixture over squash in one of the bowls; toss to coat. Arrange coated squash in one of the prepared baking pans.
In another small bowl combine the remaining brown sugar, the remaining 1/2 teaspoon salt, and 1 teaspoon ground ancho pepper. Stir in the remaining 2 tablespoons melted butter. Pour ancho pepper mixture over squash in the other bowl; toss to coat. Arrange coated squash in the other prepared baking pan.
Roast squash, uncovered, for 20 minutes. Remove from oven; stir squash. Roast for 15 to 20 minutes more or until squash is tender. Meanwhile, in a small saucepan bring half-and-half just to simmering.
Transfer one portion of the squash and the pan drippings to a large mixing bowl. Beat with an electric mixer on medium speed until smooth. Gradually beat in enough of the half-and-half to make mashed squash light and fluffy. In another large mixing bowl repeat with the remaining squash and the remaining half-and-half.
To serve, spoon one of the squash mixtures into one side of a large serving bowl and the other squash mixture into other side of the bowl; swirl together slightly. If desired, drizzle with additional melted butter and sprinkle lightly with additional ground ancho pepper.