1. Prepare the Cookie Dough:

1. In a large bowl, cream together butter and sugar until light and fluffy.
2. Mix in the egg yolk and vanilla extract until well combined.
3. Gradually add the flour and salt, mixing until the dough forms.

2. Shape the Cookies:

1. Roll out the dough on a lightly floured surface to about ¼-inch thickness.
2. Use heart-shaped cookie cutters to cut out hearts (or shape them by hand).
3. Place the hearts on a lined baking sheet.
4. Gently press your thumb or the back of a small spoon in the center of each cookie to create an indentation for the chocolate filling.

3. Bake the Cookies:

1. Preheat oven to 350°F (175°C).
2. Bake for 10-12 minutes, or until the edges are lightly golden.
3. Allow cookies to cool on the baking sheet for 5 minutes, then transfer them to a wire rack.

4. Prepare the Chocolate Filling:

1. Heat heavy cream in a small saucepan until just simmering (not boiling).
2. Pour over the chocolate chips, let sit for 1-2 minutes, then stir until smooth.
3. Mix in vanilla extract for extra flavor.

5. Assemble & Serve:

1. Spoon or pipe the chocolate filling into the indentations of each cookie.
2. Let the chocolate set for 15-20 minutes before serving.