I'm also guilty of this.
I know from my FOO (family of origin) desserts happened a lot. My dad had a sweet tooth all the time so chocolatey, ice creamy, pie like food was normal after dinner. My mother always made dinner with a meat, a veg and a starch so I think I got the idea that dessert just naturally followed that.

I also find that I feel less watched if everyone else is in bed. No one will judge what I'm snacking on if they are asleep. And if they don't know, I "don't have to feel bad about it" (never works out that way).

Not sure if either of those ideas help but I think to make it stop you need to figure out why you find yourself with less willpower at night. Stress from the rest of day built up? Tired or not tired and boredom eating? FOO issues?