Cream the butter and sugar together in a mixing bowl using an electric mixer until light and fluffy, about 2-3 minutes.
Add the egg yolk and vanilla extract, and mix well until fully combined.
In a separate bowl, whisk together flour and salt, then gradually add to the wet mixture, mixing on low speed until just combined.
Wrap the dough in plastic wrap and refrigerate for at least 30 minutes.
Preheat the oven to 350°F (175°C).
Portion out the dough using a cookie scoop and roll into 1-inch balls. Place on prepared baking sheets.
Press down in the center of each dough ball to create an indentation.
Fill each thumbprint with about 1 teaspoon of dulce de leche.
Bake for 15-18 minutes or until edges are lightly golden.
Cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.