Try overripe bananas and/or mangoes.
I have also seen "Butterfly Feeders" that consisted mainly of a NEW plastic dish scrubber in a dish of sugar water.
Where are you located, Sam?
How did you plan on overwintering a Mourning Cloak?
MCs overwinter as adults and, depending on where you are, need a winter diapause, or rest, period. They will wedge themselves into a crevice for the Winter and hibernate, not eating or moving until early Spring.
Mourning Cloaks are the first butterflies out in Spring, as they can feed on flowing sap.They can be seen with snow still on the ground.

i live in Connecticut so i have cold winters. thank you for those other choices i could feed it. it is still in its chrysilis right now though. any more tips? and what would i put in there that has a crevise for them?

Why not release it so that it can produce more Mourning Cloaks?

are they native to connecticut? its allready becoming cold. and they are still in there caccons...by the way my mc chrysilis has 3 black sparkly dots on it. one big, two small... it isnt changing color to a grayish blue like the other one...should i be worried...it looks like its kinda rotting or something.