Source: Le Viandier de Taillevent, written in the 14th by Guillaume Tirel, the chief cook of King Charles V of France.
Translation: (Translated by Terence Scully)
Toast bread slightly. Beat egg yolks. Coat toasted bread slices with egg yolk. In skillet heat oil. Fry egg coated bread about 1-2 minutes on each side until golden brown and center springs back from light touch. Lay cooked slices on plate, sprinkle with sugar and serve.
Scully, Terence, ed. The Viandier of Taillevent Canada: University of Ottawa Press, 1988