Using a food processor, blitz the flour, salt and icing sugar to combine.
Add the egg and vanilla. Blitz a few times until larger lumps start to form. It will still be quite crumbly at this stage.
Turn the pastry out onto a lightly floured surface. Quickly and gently bring it together with your hands into a smooth disc. Don’t ‘knead’ the dough - over working it makes likely to shrink back when baked and be tougher.
Pour the brittle out onto the sheet of baking paper. Place the second sheet on top, and use a rolling pin to gently spread the brittle into a flat sheet (~3mm thick.)