Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.
In a medium bowl, whisk together the flour, salt, baking powder, and baking soda.
In a large mixing bowl, use an electric mixer to cream the butter and sugar together until light and fluffy.
Sift in the cocoa powder and mix until fully combined. Then, add the eggs one at a time, mixing well after each addition. Follow with the vanilla extract and melted chocolate, blending until smooth.
Gradually add the dry ingredient mixture into the wet mixture, stirring until just combined.
Using a cookie scoop or tablespoon, portion out the dough onto your prepared baking sheet, leaving space between cookies.
Using your thumb, gently press down in the center of each cookie to create a small indentation.
Bake the cookies for 10-12 minutes or until the edges are set but the centers are still soft.
Allow the cookies to cool on the baking sheet for a few minutes before transferring them to a cooling rack. Fill each thumbprint with melted chocolate while still warm.
Let the cookies cool completely before serving.
