1. Prepare the Cookie Dough

1. In a large mixing bowl, cream butter, granulated sugar, and brown sugar until light and fluffy (about 2-3 minutes).
2. Add the egg yolk and vanilla extract, and mix until fully incorporated.
3. In a separate bowl, whisk together flour, cocoa powder, and salt.
4. Gradually add the dry ingredients to the wet ingredients, mixing until a soft dough forms.
5. Wrap the dough in plastic wrap and refrigerate for 30 minutes.

2. Shape and Create Thumbprints

1. Preheat your oven to 350°F (175°C). Line a baking sheet with parchment paper.
2. Roll the chilled dough into 1-inch balls (about 1 tablespoon each).
3. Place the balls on the baking sheet, spacing them 2 inches apart.
4. Use your thumb or the back of a spoon to gently press the center of each cookie, creating a small well.

3. Add the Chocolate Filling

1. Melt the dark chocolate chips and heavy cream together using a microwave (30-second intervals) or a double boiler. Stir until smooth.
2. Spoon a small amount of the chocolate ganache into each cookie’s well.

4. Bake the Cookies

1. Bake the cookies for 10-12 minutes, or until the edges are set.
2. Allow the cookies to cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.