ounces (100 grams) hazelnuts, shelled and skins off
ounces (100 grams) very cold butter, cubed
ounces (1/2 cup or 100 grams) sugar
ounces (3/4 cup or 100 grams) flour
ounces (100 grams) good quality dark chocolate
Start with hazelnuts, toast them gently in the oven until warm and fragrant then rub them in a tea towel to remove the skins. You don’t need to be perfect, as the skins will add a bit of flecked color to the cookies, which is lovely too. Once completely cool, grind the hazelnuts in a food processor until it resembles sand.
Like a good short crust pastry, you want to make sure your butter is very cold and you want to work quickly. Combine the hazelnut meal, cubed butter, sugar and flour in a bowl and rub with fingers until it resembles breadcrumbs (or use a food processor), then knead until it just comes together into a dough. Flatten to a disc about an inch high and place in the fridge.
When ready to make the cookies, remove from the fridge and roll out teaspoon-sized portions of dough into perfect balls and place them on a lined cookie sheet a couple of inches apart (they will spread a little as they sink into domes). Place back in the fridge for a couple of hours before baking.
Bake in a low oven at about 285 to 300ºF (140 to 150ºC) or less, if using a fan, for 15 minutes or until the cookies become hemispheres and are dry to the touch. They will be very lightly baked and extremely fragile at this point, but you do not want them to brown or melt into a puddle so watch them carefully during this time and if they seem to be cooking too quickly, remove them from the oven or turn the temperature down. Let cool completely before even touching them as they are very fragile. Once cool, they harden enough to handle.
Place the dark chocolate over a double broiler and let it melt about 75 percent. Take off the heat and stir to melt the rest of the chocolate and let it cool about 5 minutes or until it reaches a consistency where it can be spooned onto a cookie without dribbling off the sides. Place a teaspoon of the chocolate on the bottom of half of the cookies and before the chocolate sets (but not too early as it will spill and slide!) place the other hemisphere of cookie on top and leave to set.