butter cookies

1. In the bowl of a stand mixer fitted with a paddle attachment, combine the sugar, butter, egg yolk, salt, and vanilla and beat until smooth. Add the flour and mix until the dough comes together. If it doesn't, mix in 1 tbsp of water to bring it together.
2. Divide the dough in half, shape each half into a flattened disk, and wrap in plastic. Refrigerate for at least 2 hours, or overnight.
3. When you're ready to bake, preheat the oven to 350°F. Remove the dough from the refrigerator, and let it soften for about 15 minutes or until it feels soft enough to roll.
4. Lightly coat your work surface and rolling pin with flour. Roll the dough out to about 1/8" thick and cut into circles with a cookie cutter. Re-roll and cut the dough scraps. Place the cookies on parchment-lined baking sheets about 1 inch apart (they won't spread much). Pop the baking sheets in the freezer for 5- 10 minutes to firm up.
5. Bake the cookies for 12 to 14 minutes, until they're set and the edges are just beginning to brown. This can happen very quickly, so watch them closely after about 10 minutes.
6. Remove the cookies from the oven, and transfer to rack to cool completely.
7. While cookies are baking and cooling, make the ganache.

chocolate hazelnut ganache

1. Preheat oven to 350°F.
2. Toast the hazelnuts on a baking sheet, stirring frequently, until lightly browned and fragrant, about 5 minutes. Transfer to a clean dish towel, wrap tightly and let steam for 5 minutes. Rub the nuts vigorously with a towel to remove the skins then set aside to cool completely. Transfer nuts to the bowl of a food processor fitted with a metal blade and chop until finely ground (but not a paste), about 30 seconds. Set aside.
3. Place chopped chocolate in a medium bowl and set aside. In a small saucepan, heat heavy cream over medium heat until small bubbles form at the edges.
4. Pour cream over chocolate and let stand one minute for the chocolate to melt, then stir until mixture is smooth. If the chocolate doesn't fully melt, microwave cream and chocolate mixture at 10-second intervals, stirring in between, until melted and smooth. Stir in alcohol (or vanilla, if using instead) and ground hazelnuts. Cover and refrigerate until thickened but not hard (the consistency of peanut butter), about 20 minutes.
5. Stir the ganache then transfer to a pastry bag fitted with a large, round, plain tip and pipe ganache to cover the surface of the cookie leaving a slight border around the edge. Alternatively, spoon about 1 tbsp of ganache onto the cookie. Top with a plain cookie to make a sandwich, and gently press down to adhere the top cookie and squish the ganache to the edge of the sandwich. Repeat process with remaining cookies.