Step 1Mix warm milk, sugar and yeast in small bowl until yeast dissolves. Set aside.
Step 2Blend flour, baking powder, salt and lemon zest in large bowl of electric mixer. Add butter and blend until mixture is crumbly.
Step 3Beat egg, egg yolk and vanilla in small bowl. Add to flour mixture alternately with milk mixture and blend until dough comes together. Transfer dough to floured board; divide dough into 4 parts. Knead each part into flat disc, then wrap in plastic wrap and refrigerate 1 hour.
Step 1Spoon poppy seed filling into pastry bag. Blend sugar and nuts in small bowl. Set aside.
Step 2Roll each disc of dough into thin sheet large enough to fit into a 13x9-inch baking dish. Trim each sheet dough to fit dish. (Reserve trimmings for Poppy Seed Strudel, if desired.) Arrange 1 sheet dough in bottom of baking dish. Pipe half filling onto pastry (or can be spread using spoon). Spread poppy seed filling evenly over sheet of dough. Cover with second sheet of dough, and sprinkle with 1 cup walnut filling. Arrange sheet of dough on top and repeat with layer of poppy seed filling. Top with remaining sheet of dough. Cover with kitchen towel and let stand 30 minutes. Brush top with egg yolk mixture.
Step 3Bake at 350 degrees until golden brown, about 45 minutes. While hot, cut into 24 squares. Cool in pan. (To freeze, wrap each pastry in plastic wrap then foil. Store in resealable plastic bags in freezer.)