In a large mixing bowl, cream together the firm salted butter and granulated sugar until light and fluffy. This will take about 3-4 minutes.
Add the seeds of the vanilla bean (or vanilla extract) and the egg yolks to the butter mixture. Mix until well combined.
Gradually add the all-purpose flour to the mixture, stirring gently until the dough comes together. Avoid overmixing to keep the cookies tender.
Cover the dough with plastic wrap and chill in the refrigerator for at least 30 minutes. Chilling helps the cookies maintain their shape during baking.
Preheat your oven to 350°F (175°C). Line your baking sheets with parchment paper.
Take the chilled dough and scoop out tablespoon-sized portions. Roll them into balls and place them on the prepared baking sheets about 2 inches apart.
Using your thumb (or the back of a measuring spoon), create a small indentation in the center of each cookie.
In a separate bowl, whisk the egg whites until frothy. Dip each cookie ball into the egg whites, then roll them in the finely chopped pistachios, ensuring they are well-coated.
Spoon a small amount of strawberry preserves into each thumbprint indentation, being careful not to overfill.
Bake the cookies in the preheated oven for 12-15 minutes, or until the edges are lightly golden. Allow them to cool on the baking sheets for a few minutes before transferring them to a wire rack.
Once the cookies are completely cool, drizzle melted chocolate over the top for an elegant finish.
