Combine yeast and sugar with a tablespoon of warm water. Leave for a few minutes until it bubbles up.
In a mixing bowl combine yeast and sugar, flour and salt. Start adding milk slowly as you work the mixture into a ball. Continue kneading until the dough is lump and elastic. If you prefer, dust a working area and continue kneading there. When the dough is smooth, firm and elastic, cover with a kitchen towel and leave for 1 to 1.5 hours to rise.
Cut in half and knead each peace an additional 5 to 10 minutes. Roll the first piece out with a rolling pin until it's the size of a small pizza and thickness of your little finger. Cut into 8 slices. Repeat for the second dough piece.
Place half a teaspoon of jam onto the wider part of each slice and roll from larger part in. Repeat for all crescents.
Sprinkle flour onto a baking pan and place crescents onto it. Paint each with egg yolk and season with poppy seeds. Leave to rise an additional 20 minutes.
Heat oven to 395°F. Bake crescents for 15 minutes. Lower temperature to 355°F, cover with foil and bake another 10 minutes.