Preheat your oven to 350°F and line a baking sheet with parchment paper.
In a large mixing bowl, cream the softened butter and granulated sugar until light and fluffy.
Mix in the egg yolk and vanilla extract until smooth.
Add the flour and salt, then mix just until a soft dough forms.
Scoop or shape the dough into small rounds and place them on the prepared baking sheet, leaving a little space between each cookie.
Gently flatten the tops slightly with your hand or the bottom of a glass.
Brush the tops with lightly beaten egg white and sprinkle with coarse sugar, if using.
Bake for 10 to 12 minutes, or until the edges are lightly golden.
Let the cookies cool on the baking sheet for a few minutes, then transfer them to a wire rack to cool completely.