In a medium bowl, cream together the butter and 85g of flour. Divide into 2 equal parts, and roll each half between 2 pieces of greaseproof paper into a 15x30cm sheet. Refrigerate.
In a large bowl, mix together the yeast and 375g of the remaining flour. In a small saucepan over medium heat, combine the milk, sugar and salt. Heat to 43 degrees C, or just warm, but not hot to the touch. Mix the warm milk mixture into the flour and yeast along with the eggs, and lemon and almond extracts. Stir for 3 minutes. Knead in the remaining flour 60g at a time until the pastry is firm and pliable. Set aside to rest until double in size.
Cut the pastry in half, and roll each half out to a 35cm square. Place one sheet of the cold butter onto each piece of pastry, and fold the pastry over it like the cover of a book. Seal edges by pressing with fingers. Roll each piece out to a 50x30cm rectangle, then fold into thirds by folding the long sides in over the centre. Repeat rolling into a large rectangle, and folding into thirds. Wrap in cling film and refrigerate for at least 30 minutes.
Remove from the refrigerator one at a time, and roll and fold each piece two more times. Return to the refrigerator to chill again before shaping. If the butter gets too warm, the dough will become difficult to manage.
To make Danish pastries, roll the dough out to 6mm thickness. The dough can be cut into squares, with a filling placed in the centre. Fold 2 of the corners over the centre to form a filled diamond shape. Or, fold the piece in half, cut into 2.5cm strips, stretch, twist and roll into a spiral. Place a dollop of preserves or other filling in the centre. Place pastries on an ungreased baking tray, and let rise until doubled. Preheat the oven to 220 C / Gas 7. Pastries can be brushed with egg white for a shiny finish.
Bake for 8 to 10 minutes in the preheated oven, or until the bottoms are golden brown.