PreparationMake cookies: Sift together flour, confectioners sugar, vanilla powder, and salt. Heat butter with an electric mixer until creamy. Mix in flour mixture at low speed just until blended. Form dough into a disk and chill, wrapped in plastic wrap, until firm, about 1 hour. Preheat oven to 300°F. Roll out dough slightly less than 1/4 inch thick on a lightly floured surface and cut out 1 1/2-inch rounds. Arrange 1 inch apart on greased baking sheets. Reroll scraps once (cookies become tough if dough is worked too much). Whisk together powdered egg whites and water. Brush cookies with egg whites and sprinkle with sanding sugar. Bake cookies in batches in middle of oven until pale golden, 10 to 15 minutes, then transfer to a rack to cool. Fill cookies: Sandwich jam and preserves between unsugared sides of cookies, filling half with raspberry and half with apricot.
PreparationMake cookies: Sift together flour, confectioners sugar, vanilla powder, and salt. Heat butter with an electric mixer until creamy. Mix in flour mixture at low speed just until blended. Form dough into a disk and chill, wrapped in plastic wrap, until firm, about 1 hour. Preheat oven to 300°F. Roll out dough slightly less than 1/4 inch thick on a lightly floured surface and cut out 1 1/2-inch rounds. Arrange 1 inch apart on greased baking sheets. Reroll scraps once (cookies become tough if dough is worked too much). Whisk together powdered egg whites and water. Brush cookies with egg whites and sprinkle with sanding sugar. Bake cookies in batches in middle of oven until pale golden, 10 to 15 minutes, then transfer to a rack to cool. Fill cookies: Sandwich jam and preserves between unsugared sides of cookies, filling half with raspberry and half with apricot.