Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.

In a bowl, combine the ground hazelnuts, flour, and salt. Set aside.

In a large mixing bowl, beat softened butter and granulated sugar until light and fluffy (about 3-4 minutes).

Mix in egg yolk and vanilla extract until nicely incorporated.

Gradually add the dry mixture to the wet ingredients, mixing gently until just combined.

Shape small balls (about 1 inch in diameter) and create indents in the center using your thumb.

Place the cookies on the prepared baking sheet and bake for 15-18 minutes, or until edges are lightly golden.

Let them cool completely on a wire rack.

Heat heavy cream in a saucepan until it starts to simmer. Remove from heat and add chopped chocolate and butter. Stir until smooth.

Once the cookies have cooled, spoon the silky ganache into each thumbprint. Allow setting for a few minutes before serving.