  We report the first $> 99\%$ confidence detection of X-ray polarization in BL
Lacertae. During a recent X-ray/$\gamma$-ray outburst, a 287 ksec observation
(2022 November 27-30) was taken using the Imaging X-ray Polarimetry Explorer
({\it IXPE}), together with contemporaneous multiwavelength observations from
the Neil Gehrels {\it Swift} observatory and {\it XMM-Newton} in soft X-rays
(0.3--10~keV), {\it NuSTAR} in hard X-rays (3--70~keV), and optical
polarization from the Calar Alto, and Perkins Telescope observatories. Our
contemporaneous X-ray data suggest that the {\it IXPE} energy band is at the
crossover between the low- and high-frequency blazar emission humps. The source
displays significant variability during the observation, and we measure
polarization in three separate time bins. Contemporaneous X-ray spectra allow
us to determine the relative contribution from each emission hump. We find
$>99\%$ confidence X-ray polarization $\Pi_{2-4{\rm keV}} =
21.7^{+5.6}_{-7.9}\%$ and electric vector polarization angle $\psi_{2-4{\rm
keV}} = -28.7 \pm 8.7^{\circ}$ in the time bin with highest estimated
synchrotron flux contribution. We discuss possible implications of our
observations, including previous {\it IXPE} BL Lacertae pointings, tentatively
concluding that synchrotron self-Compton emission dominates over hadronic
emission processes during the observed epochs.
