1. Heat oven to 275 degrees. Brush melted butter over a 4 1/2-by-14-inch fluted tart pan with removable bottom; set aside.

2. In bowl of an electric mixer fitted with paddle attachment, cream butter and 1/4 cup granulated sugar on medium-high speed until light and fluffy, about 2 minutes. Add flour, salt and vanilla; mix, beginning on low speed and increasing to medium, until flour is incorporated. Using fingers, gently press dough evenly into pan. Using a fork, score dough evenly into five even pieces.

3. Bake until edges start to take on color but center remains pale, 45 to 55 minutes. Transfer to a wire rack to cool; unmold.

4. Place 1/2 cup granulated sugar in a saucepan. Add 1/2 cup water; bring to a boil; cook, stirring until sugar has completely dissolved. Reduce heat to a high simmer. Cut 6 plum wedges into smaller pieces; add. Reduce liquid to a thick pink syrup, 10 to 20 minutes. Strain and discard solids. Set aside.

5. In bowl of an electric mixer, whip cream and 2 tablespoons confectioners' sugar until soft peaks form, about 3 minutes.

6. To serve, top each shortbread square with whipped cream, remaining plum petals, and syrup. Dust with confectioners' sugar.