  Among condensed matter systems, Mott insulators exhibit diverse properties
that emerge from electronic correlations. In itinerant metals, correlations are
usually weak, but can also be enhanced via geometrical confinement of
electrons, that manifest as `flat' dispersionless electronic bands. In the fast
developing field of topological materials, which includes Dirac and Weyl
semimetals, flat bands are one of the important components that can result in
unusual magnetic and transport behaviour. To date, characterisation of flat
bands and their magnetism is scarce, hindering the design of novel materials.
Here, we investigate the ferromagnetic Kagom\'{e} semimetal Co$_3$Sn$_2$S$_2$
using resonant inelastic X-ray scattering. Remarkably, nearly non-dispersive
Stoner spin excitation peaks are observed, sharply contrasting with the
featureless Stoner continuum expected in conventional ferromagnetic metals. Our
band structure and dynamic spin susceptibility calculations, and thermal
evolution of the excitations, confirm the nearly non-dispersive Stoner
excitations as unique signatures of correlations and spin-polarized electronic
flat bands in Co$_3$Sn$_2$S$_2$. These observations serve as a cornerstone for
further exploration of band-induced symmetry-breaking orders in topological
materials.
