Preheat and prepare the pan. Heat your oven to 180°C (160°C fan). Line a 20cm x 20cm square baking tin with a double layer of baking paper, leaving an overhang on two opposite sides. This will be your handle later. You want the paper to sit flush against the corners, so crease it sharply.

Make the shortbread crust. In a large bowl, whisk together 250g plain flour, 75g icing sugar and the salt. Drop in the cold butter cubes. Using your fingertips, rub the butter into the flour until the mixture looks like coarse, pale sand. You should see small, pea-sized flakes of butter still visible — that’s what gives the crust its tender, crumbly texture. The mixture should feel cool to the touch and smell faintly sweet and nutty.

Press and bake the base. Tip the crumb mixture into the lined tin. Use the flat bottom of a glass or a measuring cup to press it down firmly and evenly. You want a compact, level surface with no loose crumbs. Prick the base all over with a fork. Bake for 18–20 minutes, until the edges are just turning a pale, golden brown and the centre looks dry, not greasy. The kitchen will smell of toasted butter. Set aside to cool for 10 minutes while you make the filling.

Whisk the lemon filling. In a medium bowl, whisk the eggs until they’re frothy and a pale, lemony yellow — about 30 seconds of vigorous whisking. Add the caster sugar and whisk for another minute until the mixture thickens slightly and looks glossy. The sugar should be mostly dissolved by now. Pour in the lemon juice and zest; whisk again. The mixture will look thin and smell intensely citrusy. In a small bowl, stir together the remaining 60g flour and the baking powder, then sift this over the wet mixture. Whisk gently until no white streaks remain. The batter will be smooth and runny, like a thin custard.

Pour and bake the bars. Pour the filling directly onto the warm shortbread crust. It will spread easily. Tap the tin gently on the worktop to pop any surface bubbles. Bake for 25–30 minutes. You’ll know it’s done when the edges are set and the centre barely wobbles when you gently shake the tin. The top should be a glossy, pale gold with no wet patches. If you see bubbles forming on the surface, your oven is too hot — reduce the temperature by 10°C next time.

Cool completely before slicing. Place the tin on a wire rack and let it cool to room temperature. This takes about an hour. Then, transfer the tin to the fridge and chill for at least another hour — two is better. The filling needs this time to firm up into that clean, sliceable curd. When you lift the bars out using the paper overhang, the paper should peel away cleanly. Dust generously with icing sugar just before serving.