Prepare the cookie dough:

• In a large bowl, cream the butter and sugar together until light and fluffy. Add the egg yolk and vanilla extract, mixing until smooth.
• In a separate bowl, whisk together the flour, cocoa powder, and salt. Gradually add the dry ingredients to the wet ingredients, mixing until a soft dough forms.

2. Chill the dough:

• Cover the dough with plastic wrap and refrigerate for 30 minutes to make it easier to handle.

3. Shape the cookies:

• Preheat your oven to 350°F (175°C). Line baking sheets with parchment paper.
• Roll the chilled dough into 1-inch balls. If desired, roll each ball in chopped nuts, coconut, or sprinkles for added texture.
• Place the balls on the prepared baking sheet, about 2 inches apart. Use your thumb or the back of a teaspoon to press a small indentation into the center of each ball.

4. Bake the cookies:

• Bake the cookies for 10-12 minutes, or until the edges are set. If the indentations puff up during baking, gently press them back down with a spoon while the cookies are still warm.

5. Make the ganache:

• In a heatproof bowl, combine the chocolate chips and heavy cream. Microwave in 20-second intervals, stirring until smooth. Stir in the vanilla extract.

6. Fill the cookies:

• Once the cookies have cooled, spoon or pipe the ganache into the indentations of each cookie. Let the ganache set at room temperature or refrigerate for faster setting.