Old World Poppy Seed Roll

Make the filling: Place poppy seeds into a food processor; process until seeds are ground, about 1 minute.

Mix poppy seeds, 3/4 cup sugar, hot milk, 1 tablespoon melted butter, and lemon juice together in a bowl until combined; cover and refrigerate while making bread. The filling will set up and thicken as it chills.

Make the bread: Pour warm water into a small bowl; add sugar and sprinkle yeast on top. Let stand until yeast softens and begins to form a creamy foam, about 5 minutes.

Whisk flour and salt together in a bowl. Cut in butter with 2 knives or a pastry blender until the mixture resembles coarse crumbs. Add yeast mixture and egg yolk; 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 12x16-inch rectangle.

Spread half of 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 filled loaves seam-side down on the baking sheet and allow to rise in a warm place until doubled, about 1 hour.

Preheat the oven to 350 degrees F (175 degrees C). Beat egg white in a bowl until frothy; brush loaves with beaten egg white.

Bake in preheated oven until dark golden brown on top, 30 to 40 minutes. Remove loaves from the oven and cover with a clean kitchen towel until cool to keep crust soft. Cool completely before slicing.