Here’s a tried and true recipe from the OU weekly newsletter:
DOUBLE CHOCOLATE FILLING (Pareve or Dairy)
4 ounces dairy or pareve pound or chiffon cake
3 tablespoons sugar
3 tablespoons cocoa, sifted
1 tablespoon butter for dairy, melted (use margarine for pareve)
1 large egg yolk
1/2 teaspoon vanilla extract
2 tablespoons chocolate syrup
1 tablespoon orange juice
1/3 cup chocolate mini chips
1. Process cake into fine crumbs and mix in sugar and cocoa. Add butter or margarine.
2. Beat in egg yolk and vanilla. Add chocolate syrup and juice. Fold in chocolate chips.
3. Fill hamantaschen or other cookies, strudel or Danish.
It is reprinted on this gourmet website: