Place the powdered sugar and lemon zest into the bowl of a food processor or stand mixer and blend until the zest is aromatic and well dispersed.
Add the cold butter cubes and vanilla extract, then cream the mixture thoroughly, scraping the sides to ensure uniform texture.
Introduce the flour in gradual portions, mixing until the dough is cohesive and smooth. Transfer the dough to the refrigerator, covered, and chill for 60 minutes.
Portion the chilled dough into rounded tablespoons, roll into balls, and space them approximately two inches apart on a parchment-lined baking sheet.
Create a small well in the center of each ball using your thumb, a spoon, or a rounded utensil, pressing halfway through the dough.
Fill each indentation with about ½ teaspoon of strawberry jam, and optionally dust the tops with white sparkling sugar.
Place the tray in the freezer and allow the shaped cookies to chill for 15 minutes while preheating your oven to 350°F (175°C).
Bake for 13 minutes, or until the edges just begin to turn golden. Let the cookies rest on the tray for 15 minutes before transferring carefully to a wire rack to finish cooling.