How-to Fill and Frost a Tube Cake
Cut a 1-in. slice off the top of the cake; carefully set aside. Using a very sharp knife, carve a 1-1/2-in. deep tunnel in the cake, leaving a 3/4-in. wall on all sides. Carefully remove cake and save for another use.
Spoon filling into the tunnel.
Carefully replace cake top. Frost top and sides of cake. Chill for 2 to 8 hours before serving.