Place poppy seeds into a food processor and process until seeds are ground, about 1 minute.
Mix poppy seeds with 3/4 cup sugar, 1 tablespoon melted butter, lemon juice, and hot milk in a bowl; stir to combine. Cover poppy seed filling and refrigerate while making bread (filling will set up and thicken as it chills).
Mix yeast with water and 2 tablespoons sugar in a small bowl. Allow to stand until the yeast forms a creamy layer.
Whisk flour with salt in a bowl; use a pastry cutter to cut 1/4 cup butter into the flour mixture until the mixture resembles coarse crumbs.
Pour yeast mixture and egg yolk into flour mixture and stir to make a soft dough.
Turn dough out onto a floured work surface and knead until smooth and slightly springy, about 5 minutes. If dough is too sticky, knead in more flour, about 2 tablespoons at a time.
Cut dough into 2 equal pieces. Roll each piece out into a 12×16-inch rectangle.
Spread half the poppy seed filling over each rectangle, leaving a 1-inch border. Fold the 1-inch border back over the filling on all sides and press down.
Pick up the shorter side of a dough rectangle and roll it like a jelly roll; repeat with second rectangle. Pinch ends together or tuck ends under to prevent filling from leaking out.
Line a baking sheet with parchment paper; place rolls seam sides down on the baking sheet and allow to rise in a warm place until doubled, about 1 hour.
Preheat oven to 350 degrees F (175 degrees C).
Beat egg white in a bowl until frothy; brush the rolls with beaten egg white.
Bake in preheated oven until dark golden brown on top, 30 to 40 minutes. Remove from oven and cover rolls with a clean kitchen towel until cool to keep crust soft. Cool completely before slicing.
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