Preheat the oven to 190°C/170°C fan
Mix the dry ingredients (caster sugar, plain flour, cocoa powder, and flaky sea salt) together
Melt the butter, then stir it through to form wet, clumpy crumbs
Add the egg white and mix well
Press the crumb into the 8 inch tart tin to make a 0.4–0.5cm thick crust
Bake for 25 minutes until it smells biscuity and the crumb looks set
Leave to cool completely in the tin
Optional: brush with a thin layer of melted chocolate for extra protection against creamy fillings
Store wrapped at room temperature for up to 3 days in advance
