Dissolve yeast in warm water. Set aside. Combine flour, 1/2 cup sugar, baking powder and salt. Stir in milk, 5 tbs melted butter, vanilla and yeast. Shape dough into a rectangle about 1/2" thick on a buttered cookie sheet. Cover dough with plum wedges. Brush plums with remaining tbs of melted butter. Combine 1/4 cup sugar and cardamom. Sprinkle over top of cake. Bake at 375~ 30 to 40 min, or until center tests done.