Grease sides of 9" springform pan; pour in butter and sprinkle with sugar. Cut rhubarb into 2 inch pieces. Arrange in tight rows in bottom of pan, starting with middle three rows and trimming to fit. Chop remaining pieces coarsely; sprinkle on top with orange rind, set aside. CAKE: In large bowl, cream butter with sugar till fluffy. Beat in eggs, one at a time and vanilla. Stir together flour, baking powder, baking soda, orange rind and salt. Stir half of the flour mixture into creamed mixture; stir in yogurt and remaining flour mixture. Spread carefully over rhubarb, mounding higher at edges, Wrap foil around bottom of pan and set on baking sheet. Bake in 350F oven for about 1 hour or till cake springs back when lightly touched. Let cool on rack for 15 minutes. Invert cake on serving platter. Serve warm. MAKES 8-10 SERVINGS