For chocolate sorbet: whisk sugar and cocoa in heavy large saucepan until blended. Whisk in 3 cups water; add chocolate. Bring mixture just to boil over medium-high heat, whisking constantly. Reduce heat to medium; simmer 1 minute, whisking occasionally. Remove from heat; strain into medium bowl. Cool to room temperature. Process mixture in ice cream maker according to manufacturer's instructions. Transfer sorbet to container; freeze until solid, at least 6 hours. Can be made 1 week ahead. Keep frozen.

For cranberry sorbet: combine first 5 ingredients in large saucepan. Bring to boil over medium-high heat, stirring until sugar dissolves. Reduce heat to medium and simmer until cranberries pop and are soft, stirring occasionally, about 5 minutes. Puree in batches in blender until smooth. Strain into medium bowl. Mix in grated orange peel. Cool to room temperature. Process mixture in ice cream maker according to manufacturer's instructions. Transfer sorbet to container; cover and freeze. Can be made 1 week ahead. Keep frozen.

Scoop chocolate and cranberry sorbets into dessert bowls. Garnish with orange slices, if desired.

Makes 6 to 8 servings.