Additional canned mandarin orange segments, drained and patted dry
Preheat oven to 350 F. Spray two 9-inch diameter cake pans with 2-inch high sides with non-stick spray. Using electric mixer, beat 1 1/2 cups oil and eggs in large bowl until well blended and thick, about two minutes. Add flour, sugar, cocoa powder, baking soda and salt; beat at low speed to blend. Increase speed; beat one minute longer (batter will be thick). Stir in carrots and orange peel, then orange pieces. Divide batter between prepared pans.
Bake cakes until tester inserted into center comes out clean, about 40 minutes. Cool cakes in pans 10 minutes. Turn out onto racks; cool completely.
To frost: Stir chocolate chips in heavy medium saucepan over very low heat until melted and smooth. Remove from heat and cool to lukewarm. Spoon 1/3 cup chocolate into small bowl and reserve. Beat margarine and sugar in medium bowl until fluffy. Beat in remaining melted chocolate and orange juice concentrate.
Place one cake layer on platter. Spread with 2/3 cup frosting. Top with second cake layer. Spread remaining frosting over top and sides of cake. Arrange additional orange segments around top edge of cake. Reheat 1/3 cup reserved chocolate to pouring consistency if necessary. Drizzle orange segments with chocolate. (Can be prepared 1 day ahead. Cover with cake dome and refrigerate.)