Sift the flour into a large bowl and mix in the salt.
Dissolve the yeast and sugar in the lukewarm milk and leave for about 10 minutes until foamy.
Add the yeast mixture, sour cream, 1 egg and softened butter to the flour. Knead until smooth and soft. Add more flour if needed.
Shape the dough into a ball, cover and leave in a warm place for 60 minutes.
Grind the pork cracklings into a spreadable paste and season with salt and pepper.
Roll out the dough on a floured surface (0.2 inch thick) and spread the crackling paste evenly over it.
Fold the bottom third up and the top third down over it. Then fold the left third towards the centre and the right third over it. Cover with plastic foil and refrigerate for 30 minutes.
Roll out the dough again, brush off any excess flour and repeat the folds. Cover and refrigerate for another 30 minutes.
Lay a sheet of parchment paper onto the baking pan. Roll out the dough to about 0.8 inch (2 cm) thick and lightly score the top in a crisscross pattern.
Cut out rounds with a 5 cm cutter and place them on the baking sheet, leaving space between them.
Leave to rise for 30 minutes, then brush the tops with beaten egg.
Bake in a preheated oven at 380°F, using conventional top and bottom heat, for about 25 minutes, until golden brown.