I haven't used it, but Capella Flavorings makes an Orange Creamsicle extract (online). If you're in a pinch (like I was), you can try adding (don't laugh) the duncan hines frosting creations powder in Orange Creme. I have successfully added it to my yellow cake recipe and it was delicious paired with vanilla buttercream! 
Thanks Tiz for the suggestions. Im trying to recreate our wedding cake to surprise my husband for our annaversery. It was a dreamsicle cake. I always thought of it as a white sourcream cake with the dreamsicle flavor and it had little pieces of manderan oranges in it too. But I haven't been able to find a recipe.
This is where it all starts.
http://cakecentral.com/cake-decorating-ftopict-614554.html
I buy a flavoring from King Arthur Flour called Fiori di Sicilia and it's a creamsicle flavor. It's yummy in just about everything.
Now that is some good stuff! 
To me, the flavor is more delicate than a creamsicle, but it is so good!
The recipe you describe, with the bits of mandarin orange, sounds like this one (without the nuts): http://www.bettycrocker.com/recipes/luscious-mandarin-orange-cake/02e67811-b56b-4047-8804-9cf3992dd249
Thanks everyone for your help! I've tried a couple diffrent recipes now they sure good but not what I was looking for. I'll keep experimenting, but if I don't find it I have a couple great recipes to fall back on!
For my dreamsicle cupcakes, i use a orange cake mix. (yes Im a boxed mixer) the duncan Hines Orange Supreme is divine. Instead of the veg oil it calls for i use Pamppered Chef orange infused canola oil, and grated orange if you like. The frosting i use a pure buttercream (all butter no shortening) and divide the buttercream in 1/2 after made so i can keep one side vanilla (add vanilla extract after frosting is divided), and i use orange food coloring and orange extract in the other 1/2 so i can swirl two different colors.. Everyone LOVEs THEM.
If you want to use a filling, which makes these even better try and look up the "twinkie" filling receipe.
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