Butter contains water, and a microwave heats that water most of all. When pockets of water trapped beneath the melted fat suddenly turn to steam, the butter pops and splatters inside the microwave. To prevent this, heat it at low power in 10–15-second intervals, cover the container, and cut the butter into small pieces. Taking it out and stirring when it is half melted is often enough.
Butter is not pure fat; most of it is fat, and the rest is water and milk solids. Microwave energy heats food by making its water molecules vibrate, so it targets the water most. As the butter warms, the fat melts and becomes fluid, while the water droplets inside remain trapped beneath the melted fat. Once the water temperature passes its boiling point, a droplet suddenly turns to steam and bursts out, flinging the fat above it; that is the sound you hear.
The first way to prevent popping is to lower the power. When a microwave runs at full power, the water pockets boil quickly and at the same time. Lowering the power to 30–50% slows the heating, letting steam escape before it builds up so the fat melts gently. The second way is to use short intervals: heat the butter in 10–15-second rounds, taking it out and stirring after each round. Stirring helps the hot melted fat around the edge of the container warm the cold piece in the middle. Do not extend the time in one go; two short rounds are safer than one long round.
The third way is to cover the container. Use a microwave-safe lid left slightly ajar or cover the top with a paper towel to catch splattering fat; do not seal the container completely, because steam pressure will build inside. Also cut the butter into small pieces; with a whole stick, the outside melts while the center stays cold, so it heats unevenly. Choose a deep, narrow container; in a wide, shallow container the fat spreads in a thin layer and splatters more easily. A cold container also slows heating and reduces splattering.
Most recipes do not require the butter to melt completely. If butter that is half melted, with solid pieces still inside, is taken out of the microwave and stirred with a spoon, the remaining pieces melt on their own from the residual heat and the fat does not overheat. Overheated butter browns, foams, and its milk solids darken; this is not the neutral melted butter a recipe calls for. Softening butter does not require melting it at all: cut the stick into thin slices and leave it on the counter for 15 to 20 minutes.
The alternative is the stovetop, which gives you more control. Melt the butter in a small saucepan or skillet over the lowest heat, shaking the pan occasionally; turn off the heat when the foam begins to rise. Whichever method you use, make sure the container is suitable for the microwave or stovetop; do not put a metal container or one with metal trim in the microwave. Wipe up splattered fat inside the microwave right away; accumulated fat heats up the next time it is used, develops an odor, and eventually turns into a sticky film.