Hi Debbie, If you haven't already gotten a recipe for penuche frosting, I think it might be very good on your spice cake. The recipe is from a Betty Crocker Cookbook from the 60's.

Melt butter in saucepan. Stir in brown sugar. Boil and stir over low heatr 2 min. Stir in milk. Bring to boil, stirring constantly. Cool to lukewarm (120 F.). Gradually stir in confectioners' sugar. Place pan in ice water and stir until thick enough to spread. Makes enough for two 8 or 9 inch layers or 9x13 pan.