In a small bowl, combine 100 ml of warm milk with 3 or 4 cubes of yeast and 1 tablespoon of sugar. Stir gently and let the mixture sit for about 10-15 minutes until it's frothy and bubbly.

In a large mixing bowl, combine the remaining milk, 1 kg of flour, and the activated yeast mixture, along with 1/2 tablespoon of salt. Mix well until a soft dough forms.

Knead the dough on a lightly floured surface for about 5 minutes until smooth, then place it in a warm spot covered with a cloth to rise for 45 minutes.

While the dough is rising, trim any visible veins from the 500 g of pork leaf fat and refrigerate it until firm.

Once chilled, finely shred or grind the fat to ensure it blends easily into the dough.

After the dough has risen, carefully roll it out on a floured surface until it's about 1 cm thick.

Spread one-third of the prepared leaf fat evenly over the rolled-out dough.

Once the fat is applied, sprinkle some flour over the dough, then fold it in quarters.

Wrap the folded dough in plastic wrap and refrigerate for 30 minutes.

After chilling, remove the dough and repeat the process of rolling out, spreading one-third of the remaining leaf fat, and folding the dough two more times.

Roll the dough out again until it's about 1 cm thick. Cut the dough into 8x8 cm squares.

On each square, place a spoonful of jam and a sprinkle of ground walnuts.

Fold the corners of the squares over the filling to create cute packages.

Allow the filled pastries to rest on the baking tray for 15-20 minutes.

Preheat your oven to 200°C (390°F).

Bake in the preheated oven for about 20 minutes or until they are golden brown and puffed up.

Once out of the oven, immediately dust the hot Croatian croissants with powdered sugar.