Lipids

Like sugars, lipids (fats, oils, and emulsifiers) interfere with coagulation of egg proteins and so tenderize custards much as they tenderize baked goods. Lipids likely slow coagulation by interacting directly with egg proteins, just as they tenderize gluten structure by interacting with gluten proteins.

Actually, custards made with a high amount of lipids from either cream or egg yolk are more than just soft and tender. Cream and egg yolks provide an added dimension, a smoothness and creaminess not seen in custards made without these ingredients. This creamy texture is the hallmark of well-made crème brûlée, which is custard prepared from heavy cream and egg yolks, topped with a crisp burnt sugar crust.