Place poppy seeds in a small saucepan and cover with water. Bring to a boil. Remove from the heat; let stand until cool. Strain through a fine mesh strainer.
Place poppy seeds and almonds in a food processor; cover and process until blended. In a large skillet, melt butter. Add sugar and poppy seed mixture; cook and stir over low heat for 10 minutes. Remove from the heat. Transfer to a large bowl; cool to room temperature.
Stir in the citron, egg, orange peel, honey, lemon zest and vanilla. In a small bowl, beat egg whites on high speed until stiff peaks form. Gradually fold into poppy seed mixture; set aside.
In a small bowl, dissolve yeast in warm water; let stand for 5 minutes. In a large bowl, combine 3 cups flour, confectioners' sugar and salt; cut in butter until crumbly. Add the sour cream, eggs, egg yolks, lemon zest, vanilla and yeast mixture; mix well. Stir in enough remaining flour to form a soft dough.
Turn onto a floured surface; knead until smooth and elastic, about 6-8 minutes. Divide dough in half. Roll each portion into a 14x12-in. rectangle. Spread poppy seed filling over each rectangle to within 1/2 in. of edges. Roll up jelly-roll style, starting with a long side; pinch seams to seal and tuck ends under.
Place loaves seam side down on greased baking sheets. Cover and let rise until doubled, about 45 minutes.
Bake at 350° for 30-35 minutes or until golden brown. Remove from pans to wire racks. Cool for 15 minutes. Combine glaze ingredients; drizzle over loaves.
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