Place flour and salt in a large bowl. Divide your butter/marg into 3 equal quantities (250g each). Set aside two of these to soften.
Rub or cut in the first 250g quantity of butter/marg to form a breadcrumb texture. This can also be done in a food processor on pulse.
Add the egg white, vinegar and water and mix with a fork until the mixture starts to come together. Once it becomes hard to mix, turn it all out onto the counter and knead lightly until if forms a soft, smooth dough. Set aside to rest for a few moments.
Divide the dough into 2. Roll out the dough on a floured surface (be generous with the flour).
Spread or dab 125g of soft butter/marg onto the dough and fold into thirds. Press lightly around the edges to make sure the butter/marg stays inside the envelope. Place this dough in a plastic bag or wrap and place in the fridge for 15 minutes.
Repeat this process with the other half of the dough.
With the final remaining 250g butter/marg, mix in a generous pinch of cream of tartar.
Working with your first batch, roll out on a floured surface again. Roll in one direction to form a longish oblong. Spread or dab 125g of the cream of tartar butter/marg and fold into thirds again (pinching off the sides again). Wrap and chill for a further 15 minutes.
Repeat this with the other batch.
This pastry can be used immediately or frozen. Before use I like to roll it out and fold it into to thirds a couple of extra times to ensure maximum layers and puff.