1. Preheat the Oven:

• Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper.

2. Make the Dough:

• In a large mixing bowl, beat the softened butter and sugar together until light and fluffy.
• Add the egg yolk and vanilla extract, mixing until combined.
• Gradually add the flour and salt, mixing until a soft dough forms.

3. Shape the Cookies:

• Roll the dough into 1-inch balls and place them on the prepared baking sheets, spacing them about 2 inches apart.
• Use your thumb or the back of a spoon to gently press a shallow indentation into the center of each cookie.

4. Bake the Cookies:

• Bake for 10-12 minutes, or until the edges are lightly golden.
• If the indentations puff up during baking, gently press them down again with a spoon immediately after removing the cookies from the oven. Let the cookies cool completely on the baking sheets.

5. Make the Icing:

• In a small bowl, whisk together the powdered sugar, milk, and vanilla extract until smooth. Add food coloring if desired.

6. Fill the Cookies:

• Spoon or pipe the icing into the indentations of each cooled cookie. Let the icing set for 15-20 minutes before serving.