He got the package on his birthday delivered by a postal owl. He didn't want to open it in front of the his fellow students He waited until he was alone in the dorm before he opened it.
The cake was burnt, and lopsided, the icing had been put on haphazardly. the candles looked sad perched on it. It was from Mum, she couldn't cook for anything.
This was the first year she wasn't here, watching with her anxious eyes and brittle smile. He didn't have to eat it and pretend he enjoyed it. He could just chuck it, no one would know, he really never liked eating the blasted thing. He took it out of the box, lit the candles with his wand-tip, and with little ceremony blew them out.
Then he began to eat, it ripping apart the dry burnt cake, chewing on the currants in it.
The butter cream frosting was sticky and runny on his fingers. Ignoring the note about sharing it with any new friends (he had none), he ate the entire thing himself and was sick.
Year after year when Mum sent her dreadful cake he vow to himself, that he throw it out in the end he never would. He even got them the first couple years of teaching at Hogwarts. They only stopped coming when she died. He went out bought himself a cake that year and a bottle of fire whiskey. A perfect bakery made cake. After staring for a while, he realized it revolted him, it wasn't the same. He threw the perfect cake out and drank the firewhiskey.