Cadmium chalcogenide nanoplatelets (NPLs) have gained tremendous attention in the past decade due to their extremely narrow optical features and their special electronic properties. For the application of the NPLs as optical emitters, in (photo) catalysis or sensing, further modification of the pristine (core) NPLs can be required. Therefore, many procedures for the synthesis of crowns (lateral extension of the core NPLs), shells and particle domains on the NPLs have already been developed. CdSe/CdS core/crown NPLs are characterised by extremely short photoluminescence (PL) lifetimes and high PL quantum yields (PLQYs) and might therefore be ideal candidates for lighting applications. In this work, we show how the optical properties of CdSe/CdS core/crown NPLs, especially their PLQYs, can be strongly improved by applying a sulphur precursor with a low reactivity. Under optimised conditions, the new procedure allows the growth of uniform CdS crowns around CdSe core NPLs with various lateral sizes, thicknesses and shapes. In addition, the modified growth kinetics was investigated by various methods, including UV/vis and PL spectroscopy as well as transmission electron microscopy. This journal is © The Royal Society of Chemistry.