Preheat your oven to 350°F (175°C).

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

Beat in the eggs one at a time, followed by the vanilla extract.

In a separate bowl, mix together the flour, cocoa powder, and salt.

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

Using your hands, form the dough into 1-inch balls and place them on a lined baking sheet.

Press your thumb gently into the center of each cookie to create an indentation.

Bake for 10-12 minutes, until the edges are set but the centers look slightly underbaked.

Let the cookies cool on a wire rack for a few minutes, then fill the thumbprints with chocolate.