Rub the butter into the flour. Sift the dry ingredients, flour, icing sugar and salt into a large bowl. Add the lemon zest and the cold, cubed butter into the flour and rub the butter and flour together between your fingers until it resembles a bread crumb consistency with a few pea-sized pieces of butter still visible.
Add egg and water. Make a well in the centre of the flour and pour in your egg. Using a fork, mix the egg into the flour. Add 1 tablespoon (15ml) ice-cold water and use your hands to bring the dough together. Add more water sparingly if needed.
Bring dough together and refrigerate. Tip the pastry onto a lightly floured work surface. Use floured hands to shape pastry into a ball taking care not to overwork the dough. Flatten slightly into a disc and wrap well in cling wrap. Place in refrigerator for at least 1 hour.
Roll out dough. Remove the chilled pastry from the refrigerator and transfer it to a clean, lightly floured surface. Allow pastry to sit for about 10 minutes or so to soften ever so slightly. Trying to roll pastry that is too cold will cause the edges to crack. To roll out, lightly flour your countertop and rolling pin and using firm, even strokes, roll from the centre outward, turning the pastry a quarter turn every few strokes. Roll the pastry out to the thickness of a coin (⅛” thick) and into a 12in (30cm) circle.
Prepare dough in tart tin. Flour the base of a 9in (23cm) deep fluted tart tin and gently lift the pastry and place into the prepared tin. Use your fingers to push the pastry onto the sides of the tin and into the grooves. Using a sharp knife, trim off the excess pastry from the tin. Refrigerate the prepared pastry shell for at least 1 hour to allow the pastry to relax further and butter to harden. (My preference is to leave the shell overnight in the fridge and continue with the recipe the next day. If you do this then cover well in plastic wrap to ensure that the pastry doesn’t dry out.)
Blind bake. Preheat your oven to 180°C (350°F). Once the pastry is chilled, prick the pastry base all over with a fork and then line with greaseproof paper and fill with pie weights or rice (or both) all the way up the sides. Blind bake for 15 minutes, then remove the greaseproof paper and pie weights. In a small bowl, whisk together the egg and milk to form your egg wash. Brush the base of the pastry with egg wash and return the pastry back to the oven and bake for a further 7-10 minutes or until the base is dry and a light golden colour. Remove from the oven and leave the tart base to cool.