Cream together 1 cup of softened unsalted butter and ¾ cup of granulated sugar in a large mixing bowl. Use an electric mixer on medium speed until light and fluffy, about 2-3 minutes. Beat in 1 teaspoon of vanilla extract.

In a separate bowl, sift together 2 cups of all-purpose flour, ½ cup of unsweetened cocoa powder, and ½ teaspoon of salt. Gradually add this to the creamed mixture, stirring until a soft dough forms.

Roll about 1 tablespoon of dough into a ball, then roll in 1 cup of finely chopped hazelnuts. Place on a parchment-lined baking sheet, leaving 2 inches between them.

Press a deep indentation into each cookie ball using a rounded spoon or your thumb.

Preheat oven to 350°F (175°C) and bake cookies for about 12-15 minutes. Check towards the end and gently repress the indentations if needed.

Let baked cookies cool on the sheet for about 5 minutes, then fill each thumbprint with a teaspoon of chocolate hazelnut spread.

If desired, press a whole hazelnut into the center of each filled thumbprint while the spread is still warm.