Quickly work the butter, flour, sugar, sour cream, salt and egg yolk into a dough, then divide into two equal pieces. Wrap in cling film and chill for 20 minutes.
Roll out one half of the dough thinly, to the thickness of a knife back, and place in a parchment-lined baking tray.
For the filling, beat the egg yolks with the sugar until fluffy, then add the ground walnuts and lemon zest.
Whisk the egg whites to stiff peaks, and gently fold into the walnut cream.
Spread the filling evenly over the dough in the tray.
Roll out the other half of the dough and place it over the filling, pressing the edges down lightly.
Bake in a 170-180°C preheated oven for about 35-40 minutes, until golden brown.
Serve lukewarm, cut into slices.
Based on: Malatinszky Fanny szakácskönyve (a classic Hungarian cookbook), modernised. Calories and protein are estimated.