I do not like to court controversy here, especially when it comes to important matters like confectionary methodology. However, it seems that what my fellow Americans call a “sponge cake” and what our British friends do are rather different, and one or the other of us could be wrong. We agree that the starting point is equal quantities by weight of eggs, butter, sugar, and flour. The major difference is how you combine these ingredients.
If you start by blending the butter and sugar, and then combine the remaining ingredients into a batter, you get something that could be called a “sponge cake” in the United Kingdom, but we’d call it a “pound cake” in the United States. (Some variations on this recipe, such as the Victorian sponge cake, add baking powder for extra lift and lightness.)
If you start with a meringue and then fold in the other ingredients, you get what we’d call a “sponge cake” in the United States but what would be called a “foam cake” in the United Kingdom.
So, before you enjoy your sponge cake today, you must quiz the baker to ensure that the method used to create it qualifies it for the “sponge” definition in your country of residence. You can never be too careful.