Cookie-like Au/NiO nanoparticles are grown inside a porous SiO2 sol-gel film (see figure). High-resolution TEM measurements reveal a twofold structure of the nanoparticles, showing a sharp interface with epitaxial coherence of the two phases. The nanocomposite films show a reversible change in optical absorbance in the vis-NIR wavelength range when exposed to carbon monoxide.