Beating softened butter with sugar to incorporate air, creating the leavening foundation for cakes and cookies. Sugar crystals cut into soft fat creating thousands of tiny air pockets. When baking powder generates CO2, it expands these existing pockets. The result is fine, even crumb.
Butter must be 18–21°C — soft enough to deform, cool enough to hold air. Creaming takes 3–5 minutes on medium speed — mixture should lighten in colour and increase in volume. Granulated sugar creates more pockets than powdered. Eggs added one at a time to maintain emulsion.
Press test for butter: finger should leave indent without resistance but not sink in greasy. If butter is too cold, cut into small cubes and wait 15 minutes — don't microwave. If batter curdles after eggs, a tablespoon of flour from the recipe rescues it. Cream on medium, not high — friction generates heat.
Butter too cold — won't cream. Butter too warm — can't hold air. Not creaming long enough — most people stop after 1 minute. Adding eggs too fast — emulsion breaks. Using wrong sugar for the purpose.