In a large mixing bowl, place softened unsalted butter and granulated sugar. Using an electric mixer on medium speed, cream them together until the mixture becomes light and fluffy, about 3-4 minutes.

Gently add the large egg yolk and vanilla extract into the creamed mixture. Mix on low speed until fully combined.

Gradually sift in the all-purpose flour and a pinch of salt. Mix on low speed just until incorporated, being careful not to overmix.

Form the dough into a ball, divide it in half, and shape each half into a log, about 2 inches in diameter. Wrap in plastic wrap and refrigerate for at least 1 hour.

Preheat your oven to 350°F (175°C). Unwrap the chilled dough and slice it into ¼-inch thick rounds.

Place the slices on a parchment-lined baking sheet, leaving space for spreading. If desired, sprinkle each slice with extra sugar.

Bake for 12-15 minutes, or until the edges begin to turn golden. Let them cool on the baking sheet for a few minutes before transferring to a wire rack.