Combine the powdered sugar and lemon zest in a food processor or stand mixer to release the citrus oils.

Add cold cubed butter and vanilla, then cream the mixture until smooth and fully incorporated, scraping down the bowl as needed.

Gradually add flour, mixing until the dough comes together. Cover and chill in the refrigerator for an hour.

Roll dough into tablespoon-sized balls and place them 2 inches apart on a parchment-lined baking sheet.

Use the back of a utensil, thumb, teaspoon measuring spoon, or wine cork to press a shallow indentation in each dough ball.

Fill each indentation with ½ teaspoon of strawberry jam and sprinkle with white sparkling sugar (if using).

Chill the baking sheet in the freezer for 15 minutes while you preheat the oven to 350°F.

Bake for 13 minutes, or until the edges turn light golden brown. Let them cool for 15 minutes on the baking sheet, then transfer to a wire rack to cool completely.