Dissolve fresh yeast and sugar in room-warm milk.
Add softened butter, egg and salt and mix coarsely.
Add flour in batches and knead into a soft, slightly sticky dough. Knead the dough for about 10 minutes so that it becomes elastic and smooth, and chill it, covered, for 2 hours to rise.
Fold the baking paper into an envelope about 20 cm x 20 cm, put 250 g of cold butter cut into thin slabs and roll it out with a rolling pin so that it is evenly distributed in the envelope. Put the butter plate in the envelope also in the refrigerator while the dough is rising.
On a floured work surface, roll out the dough into a rectangle about 21 cm x 41 cm (that is, slightly larger than twice the size of the butter plate).
Take the butter plate out of the baking paper envelope and place it on one side of the dough rectangle, leaving a border about 1 cm wide. Cover the butter plate with the second half of the dough and press the edge of the dough firmly on all sides.
Roll out the buttered dough into a long rectangle, fold in one of the shorter sides of the rectangle about 2/3 and its second side about 1/3 to the center, and glue the two sides together. Now fold the dough from the two folded sides together like a book, wrap it in baking paper or plastic wrap and chill it for 30 minutes.
Roll the dough sheet back out on the floured work surface into a long rectangle, fold one of the shorter sides of the square in half on the center, and place the second short side over it. Wrap the dough sheet in baking paper or plastic wrap and refrigerate for 30 minutes. Repeat this step once or twice more, depending on the recipe.
Now form pastries from the finished Danish pastry dough, depending on the recipe. Let the pastries rise at room temperature until they have visibly increased in size, and bake them.