Combine the flour and salt in mixer fitted with the dough hook
Melt the 50g of butter and leave to cool
Combine the water and vinegar
With the mixer running slowly add half the water, once absorbed in to the flour add the melted butter. Once this has mixed in at the remaining water.
Mix until the dough comes together. Remove from the mixer and kneed for about a minute.
Roll in to a ball and slash a deep cross across the top. Wrap in cling film and refrigerate for at least an hour or overnight.
Place the 400g butter between 2 bits of baking paper and hit with a rolling pin until 1cm thick. Keep it in a neat square shape as much as possible. Cover and refrigerate.
Once the dough has rested, on a floured work surface roll the dough away from the centre elongating the cross shape. Place the butter in the centre of the dough and fold in the edges so they just touch and completely encase the butter.
Using a rolling pin press firmly down on the dough to help dissipate the butter. Roll the dough, keeping the edges straight the depth of the work bench.
Once you have reached the required length fold the bottom third up and fold the top third down. Turn the dough 90 degrees so it resembles a book with the opening on the right. You have completed the first turn. Fridge for at least an hour.
You keep this pattern when completing the remaining rolls and folds always keeping the opening on the right.
For the 2nd turn, remove the dough from the fridge, with the opening on the right push down firmly with the rolling pin to get the butter moving. Keeping the short length towards you roll the dough to the depth of the work bench.
Trim any rounded edges, fold the bottom third up, and top third down. Place the trimmed edges in the dough, turn 90 degrees. Fridge.
For turns 3 and 4 repeat the process above. There should be no requirement to re-trim the edges and these should remain straight during the rolling. Fridge.
For turn 5 do as above. Fridge or freeze until ready to use.