Place the sugar, rose water, if using, and water in a heavy-bottomed pot or saucepan. A non-stick surface works best for this, or use an enameled pot, as I did. Turn heat to medium.
Once the sugar has melted and the liquid is transparent, add saffron and cardamom and mix. Bring the sugar syrup to a boil over medium heat, then let it simmer until it has thickened, about five minutes. Stir frequently.
Test if the sugar syrup has reached a "one-string" consistency: take a bit of it in the ladle, blow on it to cool it, then dip the tip of your forefinger into the syrup. Press your finger into your thumb and slowly separate them. If the syrup forms a string, like you see in the picture above, you can proceed with the next step. Otherwise continue to simmer the syrup.
Once the syrup is at the one-string consistency, quickly add the almond flour and cornstarch to it. Stir the almond flour into the syrup until thoroughly mixed. Continue stirring with the ladle for a couple of minutes - this will need some muscle work as the mixture thickens rapidly - until you have a very thick, dough-like consistency.
At this point add vanilla and a tablespoon of oil and mix it in. You are ready to take the pot off the heat when the dough easily pulls off the bottom of the pan. When you pinch off a bit of the almond dough with your fingers and form it into a ball, it shouldn't feel sticky.
Remove the hot dough to a chopping board or any surface coated lightly with oil. After a couple of minutes, as soon as the dough is cool enough to handle, oil your hands and knead it into a smooth ball. Don't wait too long because the dough will harden swiftly at this point.
On a piece of parchment paper, roll out the dough into a square about a fourth of an inch thick. You can even make it a little thinner. Set this aside to cool for about 30 minutes.
Once cooled, use a knife or pizza cutter to cut into diamond-shaped slices. The katli should slice easily.