Preheat the oven to 220C/200C Fan/Gas 7. Line 2 large baking trays.
Make a custard using 70g custard powder with 150ml boiling water - it'll be thick. Let chill in the fridge 10 mins.
Remove the pastry. If a sheet, roll out onto a floured surface. If it's a block, roll to about 2 cm thick - about 30x15cm.
With the short width facing towards you, spread the custard over the pastry. The custard should be fairly thick. Sprinkle over the chocolate chips.
Fold the pastry in half (from bottom to top - see Instagram for video) and cut into about 7 strips. Take each strip and, holding each end with one hand each, twist around once. Place on a baking tray - a few cm apart. If using, spread the egg wash over the top.
Bake for 20-25 minutes, checking to ensure they don't burn - they should be a deep golden brown.
Remove, let cool on a wire rack and I highly recommend eating whilst still warm!