  We study dark sector production in electromagnetic (EM) cascades. This
problem requires accurate simulations of Standard Model (SM) and dark sector
processes, both of which impact angular and energy distributions of emitted
particles that ultimately determine flux predictions in a downstream detector.
We describe the minimal set of QED processes which must be included to
faithfully reproduce a SM cascade, and identify a universal algorithm to
generate a dark sector flux given a Monte-Carlo simulation of a SM shower. We
provide a new tool, $\texttt{PETITE}$, which simulates EM cascades with
associated dark vector production, and compare it against existing literature
and "off the shelf" tools. The signal predictions at downstream detectors can
strongly depend on the nontrivial interplay (and modelling) of SM and dark
sector processes, in particular multiple Coulomb scattering and positron
annihilation. We comment on potential impacts of these effects for realistic
experimental setups.
