  The ionising continuum from active galactic nuclei (AGN) is fundamental for
interpreting their broad emission lines and understanding their impact on the
surrounding gas. Furthermore, it provides hints on how matter accretes onto
supermassive black holes. Using HST's Wide Field Camera 3 we have constructed
the first stacked ultraviolet (rest-frame wavelengths 600-2500\AA) spectrum of
53 luminous quasars at z=2.4, with a state-of-the-art correction for the
intervening Lyman forest and Lyman continuum absorption. The continuum slope
($f_\nu \propto \nu^{\alpha_\nu}$) of the full sample shows a break at ~912\AA\
with spectral index $\alpha_\nu=-0.61\pm0.01$ at $\lambda>912$\AA\ and a
softening at shorter wavelengths ($\alpha_\nu=-1.70 \pm 0.61$ at $\lambda\leq
912$\AA). Our analysis proves that a proper intergalactic medium absorption
correction is required to establish the intrinsic continuum emission of
quasars. We interpret our average ultraviolet spectrum in the context of
photoionisation, accretion disk models, and quasar contribution to the
ultraviolet background. We find that observed broad line ratios are consistent
with those predicted assuming an ionising slope of $\alpha_\mathrm{ion}=$-2.0,
similar to the observed ionising spectrum in the same wavelength range. The
continuum break and softening are consistent with accretion disk plus X-ray
corona models when black hole spin is taken into account. Our spectral energy
distribution yields a 30% increase to previous estimates of the specific quasar
emissivity, such that quasars may contribute significantly to the total
specific Lyman limit emissivity estimated from the Ly$\alpha$ forest at z<3.2.
