Place currants in a bowl. Cover with hot water and let stand for 5 minutes. Drain well and set aside. In a large bowl, combine the flour, sugar, baking powder, salt and baking soda. Cut in butter until mixture resembles coarse crumbs. Combine sour cream and egg yolks; add to crumb mixture. Stir in currants just until blended.
Turn onto a floured surface; knead gently 8-10 times. Divide into four portions. On ungreased baking sheets, pat dough into 4-in. circles. Cut each into four wedges, but do not separate.
Beat egg white; brush over dough. Combine sugar and cinnamon; sprinkle over tops. Bake at 425° for 15-18 minutes or until golden brown. Serve warm.