I've recently come across many results discussing the differentiation of the Moreau envelope defined by
$$
e(f)(x)\triangleq \min_{h \in H} \|h-x\|^2 + f(z)
,
$$\begin{equation}
e(f)(x)\triangleq \min_{h \in H} \|h-x\|^2 + f(h),
\end{equation}
where $f$ is a convex functional on a separable Hilbert space $H$.
Examples of results on differentiability are here, as well as Moreau's original papers. However, I didn't seem to come across any results about the twice differentiability of the operator... Are these results known/ what are the conditions on a convex function $f$ so that its Moreau envelope is twice continuously differentiable?
corrected typo mentioned in the comments and in one answer.