Preheat your oven to 350°F (175°C).
In a large mixing bowl, cream together the softened butter, granulated sugar, and brown sugar until light and fluffy.
Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.
In a separate bowl, whisk together the flour, cocoa powder, baking soda, and salt.
Gradually add the dry ingredients to the wet mixture, stirring until just combined.
Chill the dough in the refrigerator for about 30 minutes to firm it up.
Scoop the chilled dough into tablespoon-sized portions and roll into balls, then place them on a prepared baking sheet.
Use your thumb or the back of a spoon to make an indentation in the center of each dough ball.
Fill each thumbprint with your choice of chocolate chips or jam.
Bake in the preheated oven for 12-15 minutes, or until the edges are set but the centers remain soft.
Remove from the oven and let cool on the baking sheet for a few minutes before transferring to a wire rack.
