Step 1: Prepare the Dough

1. In a large mixing bowl, cream the softened butter and sugar together using an electric mixer on medium speed until light and fluffy, about 2–3 minutes.
2. Add the egg yolk and vanilla extract, and beat until fully incorporated.
3. Sift together the flour, cocoa powder, and salt in a separate bowl. Gradually add the dry ingredients to the wet mixture, mixing on low speed until a soft dough forms.

Step 2: Chill the Dough

1. Wrap the dough in plastic wrap and refrigerate for at least 30 minutes. Chilling helps the dough firm up, making it easier to shape and ensuring the cookies hold their shape during baking.

Step 3: Shape the Cookies

1. Preheat your oven to 350°F (175°C) and line two baking sheets with parchment paper.
2. Roll the chilled dough into 1-inch balls and place them on the prepared baking sheets, spacing them about 2 inches apart.
3. Use your thumb or the back of a teaspoon to gently press a small indentation into the center of each dough ball.

Step 4: Bake the Cookies

1. Bake the cookies for 10–12 minutes, or until the edges are set. The centers may still look slightly soft, which is fine.
2. Remove from the oven and use a teaspoon to gently reinforce the indentations if they’ve puffed up during baking. Let the cookies cool completely on the baking sheets.

Step 5: Make the Chocolate Filling

1. Heat the heavy cream in a small saucepan over medium heat until it begins to simmer (do not boil).
2. Pour the hot cream over the chocolate chips in a heatproof bowl. Let it sit for 1–2 minutes, then stir until smooth and glossy.
3. Stir in the vanilla extract for added flavor.

Step 6: Fill the Cookies

1. Spoon the chocolate ganache into the center of each cooled cookie, filling the indentations generously.
2. Let the ganache set at room temperature for about 30 minutes or refrigerate for faster setting.