Beat butter and salt until smooth. Add sugar and mix until incorporated. Add egg yolk and vanilla extract. Mix until combined. Add flour and stir until dough comes together.*
Cover dough with plastic wrap and allow to chill in the fridge for 1 hour.
Preheat oven to 350 degrees F. Line baking sheet with parchment paper or silicone mat.
Toss together roughly ground hazelnuts and turbinado sugar. Set aside. Whisk egg white until foamy and set aside.
Scoop tablespoons of the chilled cookie dough. Roll into a ball. Dip cookie ball into egg whites and then coat in hazelnut-sugar mixture. Place on prepared baking sheet, spacing cookie balls about 2-inches apart. Indent the center of the cookie ball with your thumb or the bottom of a wooden spoon.
Bake for 10 minutes. Remove from oven and indent the center of the cookie ball once again. Continue to bake for another 6-8 minutes until the edges of the cookie are golden brown and set.
Allow cookies to cool on baking sheet for 5 minutes before transferring to a wire rack to cool to room temperature.
To make chocolate filling: Gently melt chocolate over a double boiler. Once chocolate has melted, remove from heat. Add butter and stir until butter has melted. Allow to chocolate to cool and slightly thicken. Transfer chocolate filling to a pastry bag fitted with a large star tip. When cookies have completely cooled, pipe chocolate filling into the cookie cavities. Enjoy immediately, or allow chocolate to set up.