In the bowl of a freestanding mixer beat the flours, yeast, milk and poppy seeds together until you have a smooth elastic dough (roughly 3-4 minutes). Use a dough hook for this.
Transfer into a lightly oiled bowl and cover with clingfilm. Allow to prove in the fridge for 3-4 hours or overnight if you want to bake your croissant in the morning.
Once proved sandwich your block of butter between a sheet of greaseproof paper making sure there is plenty of overhang.
Bash the butter with a rolling pin to flatten then roll out to form a rectangle roughly 2-3mm thick and 20cmx15cm in size. Set aside then roll out the chilled dough to form a rectangle roughly 20x30cm.
Place the flattened butter onto the bottom half of the dough then fold the top half over making sure to pinch the seams all the way around. Roll out to form a rectangle 60cmx20cm.
Fold the left-hand side into the centre and repeat with the right hand side.
Now close the dough like you would a book. If the dough still feels cold you can roll it out once more and repeat this step. If it begins to feel warm wrap in clingfilm and chill in the fridge for 20 minutes.
Once you have done your second book fold wrap and chill for 20 minutes.
When the dough has rested, roll it out to a thickness of 5-7mm roughly 20cmx50-60cm. cut triangle 12cm wide and 20cm tall. You should be able to get 6-8 out of this batch.
With the narrow point of one of the triangles facing away from you, use your fingertips to stretch the two points of the triangle nearest to you out sideways so that they loosen a little.
Roll the pastry over itself, curling it into a traditional crescent shape of a croissant. Repeat the process with the remaining triangles.
Place on a baking tray lined with greaseproof making sure you allow plenty of room between them. Allow to prove for 1-2 hours or until doubled in size. Times will vary depending on the temperature of your room.
Preheat the oven to 190C. Once well risen and springy to touch brush with egg wash and bake for 20-25 minutes or until dark golden. Your whole house will smell of lovely melted butter. Allow to cool completely before filling with roasted asparagus, brie and some smoked ham and watercress.