Microwaving a boiled egg with the shell on is generally considered unsafe because it can lead to steam building up inside the egg, causing pressure to build up and potentially leading to an explosion. The shell acts as a barrier, preventing the steam from escaping, which can cause the egg to burst open.
However, if you remove the yolk from the boiled egg and microwave just the yolk, the risk of explosion is significantly reduced. Without the shell, there is no confinement for the steam to build up pressure, so it's much safer to microwave just the yolk.
That being said, you should still exercise caution when microwaving any food, including egg yolks. To microwave the yolk safely, you can place it in a microwave-safe container or a small dish. You might want to add a little bit of water to the dish to help prevent the yolk from drying out. Additionally, it's a good idea to cover the dish with a microwave-safe lid or microwave-safe plastic wrap to avoid splatters.
Keep in mind that microwaving yolks can cause them to become overcooked or rubbery quickly, so it's essential to use short microwave intervals and check the yolk's texture frequently. Stir the yolk or break the yolk sac to avoid any pressure buildup while heating.
As with all microwave cooking, be cautious when removing the container from the microwave, as it can get hot. Allow the yolk to cool slightly before handling and consuming.