Step 1: Preheat the oven to 350°F (175°C).
Step 2: Cream the butter and sugar until light and fluffy, about 2-3 minutes.
Step 3: Add the egg yolk and vanilla, mixing until smooth and creamy.
Step 4: Whisk together flour and salt, then gradually add to wet ingredients until just combined.
Step 5: Scoop tablespoon-sized portions of dough, roll into balls, and place on a parchment-lined baking sheet.
Step 6: Press your thumb into the center of each ball to create an indentation.
Step 7: Fill each thumbprint with about a teaspoon of dulce de leche.
Step 8: Bake for 12-15 minutes, until edges are lightly golden.
Step 9: Cool on the baking sheet for a few minutes before transferring to a wire rack.
