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Our understanding of dust comma encoded in our knowledge of the extinction law comma is one of the leading sources of systematic bias impacting cosmology today. We propose to measure the extinction law comma and its variation comma directly through STIS FUV and NUV spectroscopy of 21 faint white dwarf stars comma the Cosmic Flux Standards. These standards have SEDs that are exceptionally well-constrained by high S-N legacy HST panchromatic imaging comma detailed ground-based optical spectroscopy comma and temporal monitoring to assess variability. These Cosmic Flux Standards extend the CALSPEC system down to Vvirgul19.5 mag comma and has been demonstrated that measurements of these standards will allow 1% absolute photometric calibration and 0.5% relative calibration comma suitable for major upcoming projects such as the Vera Rubin Observatory and Nancy Grace Roman Space Telescope. These observations will allow a direct measurement of the extinction law by comparing the STIS spectrophotometry to the unreddened SEDs of the Cosmic Flux Standards comma allowing a population inference of the extinction law comma as well as line-of-sight estimates to each object. This probabilistic model of the extinction law will directly address dust as a source of systematic error comma and the STIS FUV and NUV observations will also extend the validity of the standards to 1 comma 150 Angstrom in preparation for future UV space-missions.