A very brief history (courtesy of my homework research).
Gateau L’opéra debuted in Paris in the early 1900s. The cake is also referred to as Gâteau Clichy after Louis Clichy who introduced the cake. Years later, another Parisian patisserie called Dalloyau reintroduced the cake as Gâteau L’opéra after the Paris Grand Opera. The cake may have been served at a French-American reception held at the opera house in the 1930s.
The practice of layering cakes, however, has a much earlier origin. Ancient Middle-Eastern cooks used to layer cakes sweetened with liquors and nut flavors. The Romans then adopted this practice when they conquered Western Europe.
Coffee and Chocolate were introduced to Europe in the 16th Century, but because of the scarcity and price of these imports, they were not incorporated widely into recipes until the 19th century. Sponge cake was also invented in the 19th century.
The cake itself is a multi-layered cake consisting of 3 layers of almond joconde sponge cake soaked in coffee-rum syrup, and alternating layers of coffee french buttercream and chocolate ganache. A chocolate glaze is then poured over the cake, and the word “opera” is traditionally written in glaze on each piece. The cake is then finished with garnish of gold leaf.
Duperret, Jean-Yves, La Nouvelle Patisserie, p. 155
Davidson, Alan, Oxford Companion to Food, p. 748