To make first layer: In a bowl, sprinkle the gelatin over the cold water and let stand 2 minutes. Add the boiling water and stir until gelatin is dissolved. Add the sugar and orange extract and stir until sugar is dissolved. Refrigerate until thickened, about 90 minutes.
Arrange orange slices in bottom of 6-cup mold and gently spoon the gelatin mixture over the slices. Refrigerate 2 hours, or until set but not firm.
Second Layer: In a large saucepan, whish together milk, cocoa, sugar, extract and zest. Bring just to a boil over medium heat, being careful not to let mixture boil over. Remove from heat.
In a seperate bowl. sprinkle gelatin over cold water and let stand 2 minutes. Add hot milk mixture and stir until gelatin is dissolved. Let mixture cool to room temperature, then gently spoon into mold over the first layer. Make sure second layer mixture is cooled sufficiently.
Refrigerate until firm, 4 hours or more. Unmold and serve.