  A measurement is presented of the $\mathrm{Z}/\gamma^{*} \to \tau\tau$ cross
section in pp collisions at $\sqrt{s} = $ 13 TeV, using data recorded by the
CMS experiment at the LHC, corresponding to an integrated luminosity of 2.3
fb$^{-1}$. The product of the inclusive cross section and branching fraction is
measured to be $\sigma(\mathrm{pp} \to \mathrm{Z}/\gamma^{*}\text{+X}) \,
\mathcal{B}(\mathrm{Z}/\gamma^{*} \to \tau\tau) = $ 1848 $\pm$ 12 (stat) $\pm$
67 (syst+lumi) pb, in agreement with the standard model expectation, computed
at next-to-next-to-leading order accuracy in perturbative quantum
chromodynamics. The measurement is used to validate new analysis techniques
relevant for future measurements of $\tau$ lepton production. The measurement
also provides the reconstruction efficiency and energy scale for $\tau$ decays
to hadrons+$\nu_{\tau}$ final states, determined with respective relative
uncertainties of 2.2% and 0.9%.
