Butter is one of the most variable sources of fat because of its relatively low melting point and ability to be plastic. For this reason, butter can be used in the muffin method if melted or in the creaming method. This variability is due to the fact that butter is not 100% fat. It actually contains 80% water with the rest of its weight coming from water. In addition, butter contains about 63% saturated fat, which seems like a substantial amount unless compared with shortening which is about 90% saturated fat.
Saturated fat is fat with no double bonds or all carbons filled with hydrogens. The molecules of these fats interact via London dispersion forces. These forces are caused by the movement of electrons within each molecule, which create temporary and instantaneous dipoles between nonpolar molecules. Because these interactions are increased by proximity, fats with more saturated fats exhibit these forces to a greater extent than those with unsaturated fats (fats with double bonds replacing some hydrogens) because double bonds in saturated fats cause nonlinear conformations of the fat molecule. The London dispersion forces make saturated fats more stable than unsaturated fats. This stability is why shortening has a much larger melting point that butter.
Butters variability in recipes is also its greatest weakness in recipes. In recipes where butter is used for creaming, the temperature must be controlled in the range of about 68-76 degrees. Any much lower than that 68 degrees and butter is pretty hard and nearly impossible to cream. Any higher than 76 degrees and parts of the butter begins to melt. (It’s important to note that melting point describes the average temperature at which a solid will completely become a liquid. The melting point of butter is actually about 90 degrees Fahrenheit, but the melting of part of the butter actually occurs at a much lower temperature.)
Despite its difficulty in many recipes, I prefer to use butter over shortenings and artificial butters because of its taste. Butter has the ability to add a lot to the flavor composition of a dish, which shortenings does not have because of their neutral taste.