Sweetheart Thumbprint Cookies

Preheat oven to 350˚F. Beat butter with confectioners' sugar, vanilla, and salt until thoroughly combined, about 1 minute. Beat in flour until a dough forms.

Roll level tablespoons of dough into balls; arrange 1 inch apart on parchment-lined baking sheets. Using the end of the handle of a flat-bottomed eating utensil or rubber spatula, press two deep indentations into the center of each ball to create a V (it will expand during baking to become a heart).

Bake until bottoms are golden and cookies are just set, 17 to 20 minutes. Meanwhile, stir together sanding sugar and a tiny pinch of luster dust. In a small bowl, beat egg white with 1 teaspoon water. Working with a few cookies at a time, brush tops with egg wash and sprinkle generously with sanding-sugar mixture.

Return to oven and bake until egg wash is dry and sugar has set, about 1 minute. Let cookies cool 5 minutes on sheets, then transfer to wire racks and let cool completely.

In a small saucepan, bring jam to a boil. Press through a fine-mesh strainer; discard solids. Spoon into heart indentations (about 1/2 teaspoon per cookie), or transfer heated jam to squeeze bottles for more precision. Let jam set completely before serving. (Cookies can be stored in an airtight container at room temperature up to 5 days.)