Is this non-separable utility? On this presentation, the last slide is titled "Non Separable Utility", and the preferences given are

$$ \frac{\left(c^\gamma (1-n)^{1-\gamma}\right)^{1-\sigma}}{1-\sigma}$$ 

However, I can log-transform them as 

$$ \log\left[ c^{\gamma(1-\sigma)} (1-n)^{(1-\gamma)(1-\sigma)} (1-\sigma)^{-1}\right]$$

which are clearly separable. Did I miss something?
You are right, these preferences are separable. A way to see it is to notice that
\begin{equation*}
U(c,n) \geq U(c',n) \Rightarrow U(c,n') \geq U(c',n')
\end{equation*}
for any $n,n'$, and 
\begin{equation*}
U(c,n) \geq U(c,n') \Rightarrow U(c',n) \geq U(c',n')
\end{equation*}
for any $c,c'$.
They are even additively separable, as your log-transformation shows.