Alfajores are widely thought to be little sandwiches made of shortbread. In fact that isn’t the case. The two little disks that contain the filling most closely resemble cakes. They contain flour, butter, leavening and eggs (cooked egg yolk). They also have one other rather unusual ingredient: cornstarch (cornflour) usually in abundance. Indeed it’s not unusual to find an alfajores recipe that contains as much cornstarch as wheat flour.
The question is: why? The answer is: gluten. Cornstarch has no gluten in it, and when it’s added to a dough in that kind of quantity it has the effect of undermining any gluten than happens to be present. The cooked egg yolks do much the same thing, the effect being extreme tenderness. This is the secret to good alfajores, which have a tooth that’s even softer than an American biscuit. Superior alfajores are so melt-in-mouth tender that they give you the impression that it’s the filling that’s holding the outsides together, not the reverse.