Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper. Cream together 1 cup unsalted butter and 3/4 cup granulated sugar until light and fluffy, then stir in 1 teaspoon vanilla extract. Gradually mix in 2 cups all-purpose flour, 1/3 cup cocoa powder, and 1/4 teaspoon salt until smooth.

Take about 1 tablespoon of dough and roll it into a ball. Roll each ball in finely chopped hazelnuts to coat them evenly.

Place the hazelnut-coated balls on the baking sheet, spacing them 2 inches apart. Press down the center of each ball using a measuring spoon or thumb to create an indentation.

Bake for 10-12 minutes. Check if they are set but still soft to touch. Allow to cool on the baking sheet before transferring them to a wire rack.

After cooling, if the indentations have puffed, gently re-press them to create a pocket for filling. Spoon about 1 teaspoon of chocolate hazelnut spread into each thumbprint and garnish with a whole hazelnut if desired.