Preheat the oven to 320°F and toast the hazelnuts until the skins begin to crack and they are warm and fragrant, about 10 minutes.
Remove the pan from the oven and let them cool until you can handle them. Then, place the hazelnuts in the center of a clean kitchen towel. Fold the corners in and rub the hazelnuts against each other to remove the skins. It's okay if some of the skins remain intact, just remove most of them.
Once cooled, add them to the bowl of a food processor, with the sugar and salt. Process the mixture until homogenous. Don't over process or you will end up with hazelnut paste.
Add the flour and cold, cubed butter. Process until the mixture is just combined and the butter is evenly distributed.
Then, add the egg and process until the mixture is homogenous. The dough will be slightly sticky.
Turn the dough out onto your work surface. Form it into a ball and divide the dough into quarters.
Roll until the logs are each about 1 inch in diameter and freeze for at least one hour.
Using a pastry cutter, or a knife, cut off 1/2" chunks and roll the dough into small balls and place them on an insulated baking sheet. These cookies will not spread much on the sheet.
Freeze the baking sheet for at least one hour before baking. The cookies should be frozen when you transfer them to the oven.
Bake at 350° F for 5 minutes, 250°F for 5 minutes and 200°F for 5 minutes. Do not use convection.
Remove from the oven and allow them to cool on the baking sheet for a few minutes before transferring to a wire rack to cool completely.
Place the semi sweet chocolate over a double boiler and let it melt about 75% or put it in the microwave to melt it. If you choose to put it in the microwave, start with 45 seconds, stir and then put it back in for 20 seconds at a time, until it's melted about 75%. Remove from the heat and stir to melt the rest of the chocolate. Let it cool until it reaches a consistency where it can be spooned onto a cookie without running off the sides.
Lay the cookies flat side up in pairs.
Using a piping bag, squeeze about a teaspoon of chocolate in the center of the bottom half of the cookie, and before the chocolate sets place the top on. Set on a cooling rack until the chocolate sets.
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