Stir together flour, baking powder and salt in a small mixing bowl; cut in butter till it resembles coarse crumbs.
In a separate bowl, add yeast to warm water, stir in egg yolks, sour cream and vanilla; blend into flour mixture; mix until soft dough forms; refrigerate dough for 1 hour.
Preheat oven to 400F; lightly grease a baking sheet; set aside.
Beat egg whites until foamy, gradually add sugar and continue to beat until stiff peaks form, add rum extract, gently fold in nuts; set mixture aside.
Divide chilled dough into 4 equal parts, refrigerating each part until ready to use.
Roll each section into a 9-inch circle on a lightly floured surface.
Sprinkle lightly with powdered sugar; cut each circle into 12 wedges.
Spread a heaping teaspoon of prepared meringue on each wedge, roll up each wedge, beginning at wide end.
Place on baking sheet and bake for 10-12 minutes or until pale golden brown.
Transfer to wire racks to cool.
Sprinkle cookies with remaining powdered sugar when cool.