Melt butter in large bowl. Set aside to cool. Meanwhile, preheat oven to 400°F (200°C) and line bottom of a square 8 X 8-in (20 x 20-cm) cake pan with parchment paper. Arrange plum halves, skin-side up, on parchment in pan. Set aside.
In large bowl, using an electric mixer, whip egg whites to stiff peaks. Set aside. Into cooled melted butter, use mixer to beat in icing sugar, egg yolks and vanilla. Set aside.
In another bowl, mix flour with baking powder. Alternately fold flour mixture and egg whites into yolk mixture, finishing with the flour mixture. Gently spread the stiff batter on top of plums. Bake on middle rack of oven 50 to 60 min., or until a toothpick inserted in the centre comes out clean.
Meanwhile, prepare sauce by simmering peaches, sugar and ½ cup (125 mL) water until sugar is dissolved and peaches are softened. Purée in blender. Spoon sauce over cake slices to serve.