This is a derivative of the original sidecar in that you use Grand Marnier as the oranger liqueur. Now the other ingredient is of course Cognac and Grand Marnier has some Cognac in it. So, a bit of sugar syrup takes the edge off too much cognac-y taste.

Cognac, lemon juice, Grand Marnier and a splash of simple syrup.

I believe the “Cadillac” name comes from Grand Marnier being referred to as “GM” which also stands for General Motors who are the manufacturers of Cadillac cars. A bit corny, if you ask me.