2 Half-fill a pot with water, bring the water to a simmer and reduce the heat to low. Set the pan or bowl containing the custard mixture over the water; the bottom of the bowl should not touch the water. Whisk the custard mixture, making sure that the water does not boil. This ensures that a gentle, even heat thickens the mixture without curdling it. Whisking traps air in the yolks for a light, fluffy mixture.