Begin by ensuring your workspace is clean and organized. Preheat your oven to 350°F (175°C) and gather all your ingredients on the counter.

In a large mixing bowl, combine the softened butter and granulated sugar. Using an electric mixer, beat them together until the mixture is light and fluffy.

Add the egg and vanilla extract to the butter and sugar mixture and mix until fully combined.

In a separate bowl, whisk together the all-purpose flour, cocoa powder, and salt.

Gradually add the dry ingredients to the wet mixture, stirring until just combined. Be careful not to over-mix the dough.

Roll the dough into small balls, about 1 inch in diameter, and place them on a baking sheet lined with parchment paper.

Using your thumb, gently press an indentation into the center of each ball.

Bake the cookies for approximately 10-12 minutes until slightly puffed and set on the edge but still soft in the center.

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

Using a spoon or piping bag, carefully fill the thumbprint indentations with chocolate ganache. Let them sit at room temperature to allow the ganache to set.