Preheat the oven to 350°F (175°C).

In a large bowl, cream together the softened butter and sugar until light and fluffy.

Add the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.

In another bowl, whisk together the flour, cocoa powder, and salt.

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

Scoop out tablespoon-sized portions of dough and roll them into balls. Place them on a baking sheet lined with parchment paper.

Use your thumb or the back of a spoon to create an indentation in the center of each cookie.

Bake the cookies for 10–12 minutes, or until the edges are set but the centers are soft.

Remove from the oven and let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack.

Once cooled, fill each thumbprint indentation with chocolate ganache or fruit preserves.