Trim the roots and the togh green stalks from the leeks and remove the outer layer. Cut the leeks in half lengthways and run under cold water to rinse, peeling back the layers to get inside where the grit is lodged.
Slice the leeks crosswise into 4 mm slices and drop into a bowl of cold water for about 10 minutes. Any remaining dirt will fall to the bottom, Scoop out the leeks with a slotted spoon then drain in a colander. Pat dry.
Heat the 25g of butter and the oil in a heavy bottomed frying pan over a medium low heat. When the butter starts to foam add the leeks, flaky sea salt and the black pepper and saute for 10-15 minutes until soft but without colour.
When they are cooked, tip into a bowl, allow to cool then chill in the fridge for 20 minutes. This can be done up to a day in advance.
Line a baking tray that will fit into your fridge with baking parchment.
In a medium bowl weigh out the parmesan, flour and baking powder. Stir together well, then cut in the butter with the back of a fork ( or use a mixer) until crumbly.
Add the yoghurt, ham, fine sea salt and the cooked leeks and mix quickly to combine. Pat into a cube and place on a lightly floured surface.
Pat the dough into a thick log and cut out about 12 triangles from the log. Place the scones on the lined baking tray and put in the fridge to chill until set. This will take about an hour.
Preheat the oven to 180 C, 160 Fan, Gas 4. Line a new baking tray with baking paper and place the scones on it. Brush the scones with the egg wash and bake for 25 to 30 minutes until golden, then serve immediately.