“My my! Don't you look dapper?”
Your brother is getting ready to go out on a date. When he comes out of his room, he's dressed nicely. You say this to him.
My my! Don't you look dapper?
"My, my!" means something like "Wow. I'm impressed."
However, English speakers rarely use "My, my!" when they're honestly impressed. More often, they say "My, my!" to tease someone. For example, if someone in your family usually wakes up late, you can say this when they wake up earlier:
My, my! You're up early.
Although we use "My, my!" to tease people, it doesn't seem mean or offensive. It's fun and playful.
Use the phrase "Don't you ___!" to mean "You really ___. Wow!" For example:
Don't you smell nice!
This only works with positive things; you can't say things like "Don't you stink!" This phrase is especially used to compliment how someone looks.
You stress the word "you" when you're using this phrase. Although the sentence is formed as a question, you should pronounce it as a statement, not a question.
You can also use "Aren't you (something)!" in the same way:
Aren't you smart!
"Dapper" is an old-fashioned word that means "handsome". You call a man "dapper" when he's clean, dressed nicely in a formal way, has a neat and stylish haircut, and so on.
Aside from calling a person "dapper", you can also say that a man's clothes are "dapper":
He wore a dapper white suit.
The word "dapper" sometimes appears in the combination "a dapper gentleman".