The Crêpe au Sucre is the most reduced form of the French sweet crêpe and, by some distance, the most common. A thin wheat-flour batter of milk, eggs, butter, flour, and a pinch of salt is poured onto a hot crepière, spread with a wooden rake, cooked until the underside is lightly browned, flipped briefly, then dusted with white granulated sugar and folded into quarters. The whole transaction at a Parisian street cart takes under ninety seconds. The paper cone goes into one hand, the crêpe gets eaten on the walk, and the sugar dissolves into the residual butter on the surface as it goes.
The sandwich-like quality is in the fold rather than the filling. The crêpe is folded once into a half-circle, then again into a quarter triangle, with the sugar trapped between the layers; the form is closer to an envelope than to a wrap, and it is eaten point-first. The minimal filling is the point of the form. There is no chocolate, no fruit, no whipped cream getting in the way of the bread, and the buttered surface of the crêpe gets to do most of the flavour work. A good crêpière reads which side of the crêpe is the prettier one (the first side cooked, with the lacy edges and the irregular browning) and folds with that side facing out, so that what you see and what you eat first is the surface that crisped against the iron.
It is the dessert that French children associate with school holidays, the Chandeleur (the February 2 holiday when crêpes are eaten by tradition), and the after-school visit to the boulangerie. The Crêpe Beurre-Sucre adds a knob of cold butter to the same arrangement and produces a more luxurious version of the same idea, and the variations that follow it (jam, chestnut cream, fruit, chocolate) extend the form into the full sweet-crêpe repertoire. The wider Breton-and-national Crêpe & Galette Salée tradition covers the buckwheat savoury cousins and the more elaborate sweet fillings, and the Crêpe au Sucre sits at the absolute simplest end of that family: one batter, one folding pattern, one ingredient, and a paper cone.