Dissolve yeast in water. Set aside. Combine flour, 1/2 cup sugar, baking powder and salt. Stir in milk, 5 tablespoons melted butter, vanilla and yeast mixture.
Pat dough into a 11x12-inch rectangle on lightly buttered baking sheet. Arrange plum wedges slightly overlapping to cover top of dough. Brush plums with remaining 1 tablespoon melted butter. Combine 1/4 cup sugar and cardamom. Sprinkle evenly over top of cake.
Bake at 375 degrees 30 to 40 minutes, or until center tests done. Cool to warm and serve.