Eat the Chocolate Bar

Haskell, 101 bytes

Haskell has never been great for code golfing...

b="\n|__|__|"
h="   ____\n "
f 1=h++"  |__|"
f 2="_______"++b
f 3=h++"__|__|"++b
f n=f(n-2)++b
f _=""

JavaScript ES6, 80 bytes

x=>(x%2?`   ____
 ${x<2?"  ":"__"}|__|`:x?"_______":"")+`
|__|__|`.repeat(x/2)

Uses the same technique as my Japt answer, and is surprisingly not much longer...