Method

• 1.
  For the shortbread pastry, combine the flour, baking powder and a pinch of fine salt in a food processor fitted with the steel blade and whiz so they’re thoroughly mixed. Tip into a bowl and set aside. Put the butter in the food processor along with the caster sugar and whiz for about 40 seconds, or until the mixture is pale and creamy (you may need to stop and scrape down the side once or twice). Add the egg yolks and vanilla, then process again for 15 seconds, or until well combined. Add the flour mixture and pulse in short bursts until a ball of pastry forms around the blade (try not to overdo this or the pastry may be a bit tough).
• 2.
  Very lightly dust a work surface with extra flour. Turn the pastry out onto the work surface and shape it into two equal logs (if the pastry seems too soft, refrigerate it for a little while, so it firms up enough to handle comfortably). Wrap each log in baking paper, then refrigerate for at least 3 hours (or overnight), until really firm.
• 3.
  Meanwhile, for the lemon curd, combine the eggs and caster sugar in a medium heavy-based saucepan and whisk until thoroughly mixed but not too frothy. Whisking gently, mix in the lemon juice and melted butter. Set the pan over medium-low heat. Cook the mixture, stirring constantly with a flat-based wooden spoon or sauce whisk, until it thickens to a custard-like consistency. As you stir, try to use a figure-of-eight motion to cover the entire base of the pan, so the curd doesn’t catch and burn. The most important thing of all is not to let it boil, or it may curdle. As soon as it’s ready, remove from the heat and strain through a fine sieve into a heatproof jug. Stir in the lemon zest, then cover the jug loosely with a sheet of baking paper and leave to cool. Once cool, cover jug with biodegradable plastic wrap, pressing wrap to curd surface, and pop in the fridge. The curd will keep well for up to 10 days.
• 4.
  Preheat oven to 180°C/160°C fan-forced. Lightly butter a 25cm loose-based tart pan.
• 5.
  To assemble the tart, remove one pastry log from the fridge. Coarsely grate the pastry onto a plate. I tend to do this in batches, as the grater fills quickly and the pastry strands will squash together if they become too compacted. As you finish each batch, carefully transfer the grated dough to the prepared pan. When you have finished grating, gently pat the dough strands out evenly over the base of the pan, trying not to squash them down too much. Dollop the lemon curd evenly over the grated pastry. Smooth it out into a thin layer using an offset spatula, leaving a 1cm border around the edges. Grate remaining pastry log the same way as before and scatter it over the lemon curd. Give it the lightest pat down to even it out.
• 6.
  Carefully transfer the pan to the oven (watch out you don’t pop the base up – unfortunately, it’s quite easily done) and bake the tart for about 45 minutes, or until deepish golden brown. Check after half an hour, as it can go quicker in some ovens. Remove it from the oven and cool completely in the pan.
• 7.
  To serve, carefully remove tart from pan (see note) and place on a serving plate. Dust with icing sugar and serve with softly whipped cream.