To make the dough, put the flour, sugar, salt, yeast and lemon zest into a large bowl and mix together. Put the milk and butter into a small pan and heat gently until the butter has melted, then set aside to cool to lukewarm. Add the milk mixture and 1 egg to the flour mixture, gradually, and mix to form a shaggy dough (you may not need all of the liquid). Knead until the dough is smooth and elastic (10-15 minutes). Put the dough into an oiled bowl, cover with clingfilm and allow to rise for about an hour, or until doubled in size.
To make the filling put the butter and chocolate into a small, heatproof bowl set over a pan of simmering water, heating until fully melted. Remove from the heat and use a small whisk to mix in the icing sugar and cocoa powder. Set aside until thickened to a spreadable consistency.
Put the dough on a lightly floured worksurface and roll out into a rectangle, roughly 40 x 60cm. Spread the filling over half of the dough and top with the pistachios and sea salt flakes. Fold the uncovered dough over the filling and gently press together. Using a sharp knife or a pizza cutter, cut into thin strips between 2 and 2.5cm thick. Twist each strand of dough a few times then, gripping one end between your thumb and forefingers, coil the dough around the fingers to form a bun shape, finishing by folding the end of the dough over and under.
Put each bun onto a baking paper-lined tray, leaving 5cm of space between each bun. Cover the tray lightly with a piece of clingfilm and set aside until risen and puffy (about 45 minutes - 1 hour).
Heat the oven to 190C/fan 170C/gas 5. When ready to bake, brush the buns with beaten egg. Bake for 20 minutes or until golden brown.
Make the glaze by boiling the sugar, together with 75ml water, for a couple of minutes. Remove from the heat and add the vanilla. While the buns are fresh from the oven, brush liberally with the glaze, sprinkling with a few extra chopped pistachios. Store in a sealed container for up to three days.
