In a food processor, place the toasted pistachios and pulse until finely ground.
Add the flour and sugar and pulse for about 15 seconds.
Cut the cold butter into 1/3" cubes then add to the flour mixture. Pulse about 5 times (3 seconds each pulse) or just until the butter has incorporated.
Add the yolk and milk and process to incorporate.
Place the dough on a clean flat surface and bring it together using your hands to form a dough disk.
Place the dough between two pieces of parchment paper and roll into 1/3" thick.
Place in the refrigerator and let cool for about 30 minutes.
Preheat oven to 350F and line 2 cookie sheets with parchment paper.
Remove dough from the refrigerator and use 2" flour cookie cutter to cut cookies. Place half of the cookies in the lined cookie pan.
Bake for about 8-10 minutes until the sides of the cookies get a brown golden color.
While the first batch bakes, place the second half in the second lined cookie pan and let the cut cookies soften for 10-15 minutes.
Use 1/4" round cookie cutter to cut the center of now softened cookies.
Remove baked cookies from the oven and place the second pan to bake.
Over the stove, bring heavy cream to a boil.
In a wide and deep pan, place the sugar, cream of tartar and water.
Place over the stove and cook medium-high heat. DO NOT STIR!
The sugar will start to bubble from the sides of the pan towards the middle. If you notice some sugar crystallization on the sides of the pan, brush the sides with a lightly wet pastry brush.
Once the sugar gets a light amber color carefully add the hot cream while stirring with a rubber spatula. (Before adding the cream, if you notice the sugar is getting darker on one side and not the other, turn the pan, but DO NOT STIR!)
Remove from heat and add the butter and vanilla extract, stir until completely melted.
Add the Orange Blossom Water, stir to distribute.
Place in a glass container and let cool.
Once the caramel reaches the ideal consistency, spread 1 tsp on a cookie and top it with another cookie.