Weigh or measure the confectioners' sugar, then set aside in a bowl. In another bowl, combine the all-purpose flour and salt and whisk to blend evenly. Preheat your oven to 350°F (175°C or 180°C) so it's ready for baking later.

Blend the softened butter and the measured confectioners' sugar together in a mixer (either hand or stand) for 2-3 minutes until smooth. Stop the mixer and scrape down the sides as needed. Add in the egg whites and vanilla extract, mixing again until the mixture is well blended. Slowly add the flour and salt mixture to the wet ingredients and mix until fully incorporated, about 1 minute. When finished, remove the dough from the mixer and ensure all pieces from the bottom are mixed in.

Line cookie pans with parchment paper. Check the dough's consistency; if it's too stiff for piping, microwave half of the dough for 2-3 seconds to slightly soften (be careful not to melt the butter). Place half the dough in a 16-inch piping bag fitted with a large open star tip. Pipe 12 cookies onto the prepared pan, making each about 2 inches in diameter. To ensure the cookies hold their rosette shape, refrigerate the piped dough for 15 minutes if it's soft. I find this quick chill really helps the cookies keep their beautiful pattern during baking.

Using the round end of a 1/8" teaspoon, gently press a small indent into the center of each piped cookie. Fill the indents with a small amount of thick fruit jam. This adds a lovely burst of flavor and a bit of gleaming color to each cookie.

Bake the cookies in the preheated oven for 9-11 minutes, or until the sides are just beginning to turn golden brown. Immediately transfer the baked cookies to a cooling rack so they stop baking and cool evenly. I like to let them fully cool before enjoying, so the jam sets nicely and the texture is at its best.