Bring the fresh yeast, eggs and butter to room temperature. Crumble the yeast into a large bowl, add warm (lukewarm) milk and mix. Add 1 tablespoon flour and 1/2 teaspoon sugar. Mix, cover with a cloth and set aside in a warm place without drafts for 15 minutes until the yeast froths.
Sift the flour into the risen yeast and stir with a wooden spoon. Beat the eggs with sugar and salt and add to the bowl with flour. Mix with a spoon and add pieces of soft butter. Mix and knead the dough long and vigorously, lifting it up and kneading it several times (you can knead the dough by hand in a bowl, on a counter or with a mixer attachment suitable for yeast dough). The dough will be ready if it easily separates from your hands, it can be quite liquid and sticky. After kneading, place the dough in a bowl, cover with a cloth and set aside in a warm place to rise for about 30 minutes.
Knead the dough on a counter or work surface for about 2-3 minutes. Place it in a larger bowl, cover with a cloth and place in the refrigerator for 12 hours.
Knead the dough again (about 3 minutes), roll out on a floured counter into a rectangle about 1 cm thick (about 24 x 36 cm in size). Divide the dough into 2 strips, 12 cm wide each. Divide the strips into 3 squares, 12 cm per side, and cut them diagonally into 2 triangles. Place 1 teaspoon of jam in the center of each triangle and roll into a small spiral, starting from the longer side. Form a croissant shape.
Arrange the croissants seam-side down on two baking sheets lined with parchment paper. Leave spaces between the croissants. Brush the croissants with beaten egg and set aside for about 30 minutes until they rise.
Preheat the oven to 180°C. Brush the croissants from the first baking sheet once more with egg and bake for about 15 minutes until they lightly brown on the top and bottom. Remove from the oven and after a moment transfer to a metal rack. Bake the second batch of croissants. Dust with powdered sugar.
