Quickly rub together the cold butter, flour, sugar, salt and egg yolk, then add enough sour cream to form a firm, elastic dough. Wrap in cling film and chill for 20 minutes.
Meanwhile prepare the poppy seed filling: put the sugar cubes, water, lemon zest and lemon juice in a pan, bring to the boil, then stir in the ground poppy seeds. Cook over low heat, stirring constantly, for 5-6 minutes, until you get a thick paste. Stir in the butter and let cool completely.
Roll out the dough on a floured surface into a rectangle about 3-4 mm thick.
Spread the cooled poppy seed filling evenly over the dough, leaving about 1 cm bare at the edges.
Roll up the dough tightly from the long side into a flat log, and place seam-side down on a parchment-lined baking tray.
Brush the top with beaten egg, then sprinkle with the almonds mixed with granulated sugar.
Bake in a 180°C preheated oven for about 30-35 minutes, until golden brown.
Let cool slightly before slicing.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.