Mmmmm, I'm not familiar with opera cream filling.
Did find opera cake and opera torte recipes:
http://www.scena.org/lsm/sm7-8/cake.html
http://www.epicurious.com/recipes/recipe_views/views/108593
http://splendidtable.publicradio.org/recipes/dessert_opera.shtml
http://www.dianasdesserts.com/index.cfm/fuseaction/recipes.recipeListing/filter/dianas/recipeID/2012/Recipe.cfm
(large quantity recipe)
http://www.pastrychef.com/htmlpages/recipes/operatorte.html
Might need to clarify with customer, what an opera cream filling is (to him/her).
HTH
Easter Egg w/cream filling recipes:
(could be adapted to plain cream fillings)
http://www.unbred.com/rec/1/b/easter/189.htm
http://kids.cdkitchen.com/recipes/recs/536/Easter_Egg_Candies58879.shtml
http://www.sassycats.com/easter_recipes.html
HTH
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