Since Harlan has missed the point that a finished and decorated cake is lovely to behold and that the process of beginning to eat it, even a little bit of it, destroys the beauty of the whole cake, I guess we need to be annoyingly specific and wordy in our witty sayings…
“You can’t have a cake whole and uneaten and start eating it too.” or…
“You can’t eat your cake and still have it to eat.”
“Look before you leap if you don’t know where you’re going.”
“A penny saved is a penny earned, but investing it earns even more.”
“Let sleeping dogs lie unless you need to move the mutt.”
There, happy now Harlan?