In a small thick bottomed saucepan on low heat, melt 1/2 cup sugar stirring often until dark brown (I leave it untouched at first, until the sugar starts to melt around the edges, after that I keep a close eye on it).
Very slowly, taking care to stand back, pour 1/3 cup boiling water, whisking constantly into the melted sugar (the mixture will bubble and steam quite a bit).
Once combined, the mixture will turn very dark brown.
Add remaining 1/2 cup sugar, and simmer until completely melted.
Remove from heat; add 8 tablespoons butter (it will melt very slowly). The color will lighten. Leave for a few minutes.
Whisk 3 egg yolks. While whisking constantly, very slowly add the caramel mixture to egg yolks.
Add the rest of the ingredients. Knead, at first with a spoon, then your hand, until the dough resembles play dough, which is just slightly sticky. If the dough is very sticky, add the extra 1/4 cup rye flour.
Cover or wrap, and leave in refrigerator for at least 20 minutes, and up to 24 hours.
When ready to bake, allow the dough to warm up. It will be beautifully pliable, without any hint of stickiness.
Preheat oven to 350ºF and prepare two cookie sheets lined with parchment paper.
Roll the dough out to about 1/8 of an inch, very thin (no need to use extra flour). Cut out your favorite cookie shapes.Bake for 8-10 minutes, until the edges start browning just a little. Cool on wire rack before glazing.
