A heat sink has as its purpose ABSORBING heat which may (usually) be created by the heat source (light bulb, element etc) at a rapid but fluctuating rate and then releasing that heat back into the environment slowly and steadily. A practical example is the copper bottom on many sauce pans and broilers. The copper conducts/radiates heat at a very steady, constant rate and within a limited temp range, protects what you are cooking from spikes in temp, or uneven heating on different parts of the pan bottom. Small canning jar(s) filled partially or fully with water and lids tightly sealed (to limit/prevent increased humidity) work acceptably. Some types of stone work really well, too...