  Let $\Omega \subseteq \mathbb{R}^n$ be a bounded open $C^{1,1}$ set. In this
paper we prove the existence of a unique second order absolute minimiser
$u_\infty$ of the functional \[ \mathrm{E}_\infty (u,\mathcal{O})\, :=\, \|
\mathrm{F}(\cdot, \Delta u) \|_{L^\infty( \mathcal{O} )}, \ \ \ \mathcal{O}
\subseteq \Omega \text{ measurable}, \] with prescribed boundary conditions for
$u$ and $\mathrm{D} u$ on $\partial \Omega$ and under natural assumptions on
$\mathrm{F}$. We also show that $u_\infty$ is partially smooth and there exists
a harmonic function $f_\infty \in L^1(\Omega)$ such that \[ \mathrm{F}(x,
\Delta u_\infty(x)) \, =\, e_\infty\, \mathrm{sgn}\big(f_\infty(x)\big) \] for
all $x \in \{f_\infty \neq 0\}$, where $e_\infty$ is the infimum of the global
energy.
