This was a book I read in the 70s, but it might have been written anytime after WWII and it had a certain 50s or 60s vibe to it.
I am pretty sure it took place during WWII and was set somewhere in Europe.
I recall the character — a boy (I think) of around 12 — had a couple squares of his chocolate ration left. He was very hungry and they didn’t have much food, but he gave his ration away to another child — I think a girl, around the same age.
For the longest time I thought it was from Snow Treasure by Marie McSwigan but I tracked down that book and read it to my son and there is no such scene.
This could be Twenty and Ten, also published as The Secret Cave, by Claire Huchet Bishop. It’s set in France during WWII. The main plot is about a group of Catholic children hiding a group of Jewish children, but the scene of the boy sharing the one square of chocolate with the girl stands out in my memory too.
Yes, you’re thinking of Twenty and Ten by Claire Huchet Bishop. Coincidentally, I just reread this book last month!