Backstory: There's a type of cupcake type thing here in Europe called a Madeleine or Magdalena. It's what they say inspired Proust to write A La Recherce du Temps Perdu. Kind of lemony flavoured.
I feel quite certain I could make my own, but the really good ones are covered in a special kind of sugar. It's regular sugar, only the crystals are much bigger and square-shaped. Kind of like coarse sea-salt (only sugar, of course). It's kind of shaken all over the tops and it makes them kind of crunchy-sweet while still retaining the square shape of the crystals.
Does this have a special name among people who bake a lot? What should I look for? Or is it only something  that professionals have access to?

(Don't laugh; it is nearly effing impossible to get cream of tartar here unless you are a vinegrower, a pharmacist or a professional baker. They don't sell it in shops and it comes in 2 or 5 kilo bags only.)