In a large bowl, cream butter and sugar until light and fluffy. Beat in egg yolks. Combine the flour, baking powder and salt; gradually add to the creamed mixture and mix well. Press onto the bottom and 1 in. up the sides of a greased 9-in. springform pan. Place pan on a baking sheet. Bake at 400° for 10-12 minutes or until lightly browned. Cool on a wire rack. In a large saucepan, gently toss the blueberries, 1/2 cup sugar, tapioca, cinnamon, lemon peel and nutmeg. Let stand for 15 minutes. Cook, uncovered, over medium heat until mixture comes to a boil. Cook and stir for 2 minutes. Remove from the heat; pour into crust. In a large bowl, beat the egg yolks, sour cream, vanilla and remaining sugar. Spoon over blueberry mixture. Return pan to baking sheet. Bake at 350° for 35-40 minutes or until center is set. Cool on a wire rack for 10 minutes. Carefully run a knife around edge of pan to loosen; cool 1 hour longer. Refrigerate for at least 4 hours before serving. Refrigerate leftovers. Yield: 10-12 servings.