A Double Boiler Keeps Heat Below a Boil
A double boiler — a bowl set over (not touching) a pot of simmering water — gently heats delicate mixtures like melting chocolate, making custard, or cooking egg-based sauces with indirect, more even heat capped near water's boiling point, protecting ingredients that would scramble, seize, or scorch over direct high heat. The water underneath should stay at a bare simmer, not a rolling boil — too much heat defeats the purpose and can still overheat what's above it.
Sources & methodology: These tips draw on established food-science references —
Harold McGee's food-science writing, America's Test Kitchen /
Cook's Illustrated testing, Serious Eats'
technique reporting, USDA food safety guidance,
and peer-reviewed food-safety research (including UC Davis
cutting-board studies) — not reworded recipe-blog consensus.