To make the Pastry, cream the butter and sugar together until the mixture is a pale yellow. Add the egg, if using, then fold in the flour, mixing quickly until it is just absorbed. Alternatively, add the flour to the creamed butter and sugar and pulse quickly until it is combined. Do not over-mix. The pastry will be soft and paste-like at this stage and requires chilling for 1 hour before use. When ready to use the dough, remove the pastry from the refrigerator and turn it out onto a lightly floured work surface. Cut or break the dough into pieces and knead it lightly to soften. Roll out to an even thickness to ensure even shrinkage when baking.
To make the Filling, begin by soaking the wheat in cold water for 24 hours. The next day, bring a large pot of water to the boil with 1⁄2 teaspoon salt. Add the wheat and boil for 1 hour. Drain. In a food processor, pulse the wheat to crack open the outer shell (the bran). Do not blend to a paste – the wheat should still be in quite large pieces, which helps it absorb the milk more readily as it cooks. Put the wheat into a heavy-based pan with the milk, cinnamon, vanilla bean and seeds and lemon zest. Cook slowly for 1–11⁄2 hours, until the milk has been almost completely absorbed. Watch the pan and stir occasionally to make sure the milk does not catch on the bottom and burn. Remove from the heat and leave to cool. Refrigerate overnight, or for at least 8 hours, to allow the flavours to intensify and any excess milk to be fully absorbed. The next day, preheat the oven to 180°C and line a baking sheet with baking paper. Cut out a 26 cm circle of sweet shortcrust pastry (to fit the base of the 26 cm springform tin), and bake on the lined baking sheet for 12–15 minutes, or until golden brown. Remove from the oven and leave to cool. Fit the pastry round into the greased springform tin and clip it tight. Press the remaining dough into the tin so that it fits into the sides. Ease the pastry up the sides to a height of around 5 cm. You will need to smear it onto the base to create a tight seal. The pastry shell should be around 4–5 mm thick. Trim the edges neatly. Line the pastry with foil or non-stick baking paper, ensuring that the lining covers the base and comes up and over the sides. Fill the pastry shell with baking beans (or use dried beans or rice). Ensure that the bean filling supports the walls of the tart. Bake for about 30 minutes, or longer if you notice any opaque grey patches. While the pastry is blind-baking, prepare the filling. In an electric mixer, whisk together the egg yolks and sugar. Change to a beater attachment and blend in the ricotta until it is well combined. Remove the aromatics from the cold cooked wheat and fold into the filling with the candied peel. Whisk the egg whites until they form soft peaks. Add the 2 tablespoons caster sugar and continue whisking to form stiff peaks. Fold the egg whites into the ricotta filling and pour the mixture into the prepared pastry shell, spreading evenly. Return the tart to the oven and bake for 50–60 minutes. When cooked, the centre should be firm when pressed lightly. Leave to cool then dust with icing sugar. The tart should be served at room temperature.