Preheat and Prepare: Preheat your oven to 180°C (160°C fan). Grease and line a 23cm (9-inch) square baking tin with parchment paper, leaving some overhang on the sides to make it easy to lift the bars out later.

Make the Crumb Mixture: In a large bowl, whisk together the 280g plain flour, 200g caster sugar, baking powder, and salt. Add the cold, cubed butter and use your fingertips to rub it into the flour mixture until it resembles coarse breadcrumbs. Stir in the single egg yolk until the mixture starts to clump together.

Form and Bake the Base: Take a little more than half of the crumb mixture and press it firmly and evenly into the bottom of your prepared tin. I find using the back of a spoon helps to create a flat, compact layer. Bake for 15-18 minutes, or until the edges are just beginning to turn a light golden brown.

Prepare the Lemon Filling: While the base is baking, make the filling. In a separate bowl, whisk the 3 large eggs and 200g caster sugar together until pale and slightly thickened. Whisk in the 30g plain flour, followed by the lemon zest, lemon juice, and double cream. Mix until you have a smooth, combined batter.

Assemble the Bars: Once the base is out of the oven, carefully pour the lemon filling over the hot crust. It should spread out evenly.

Add the Topping: Crumble the remaining crumb mixture evenly over the top of the lemon filling. It's okay if there are some gaps.

Bake Again: Return the tin to the oven and bake for a further 20-25 minutes. The bars are ready when the filling is set around the edges with only a slight wobble in the very centre, and the crumb topping is a lovely golden brown.

Cool Completely: This is a crucial step! Allow the bars to cool completely in the tin at room temperature for at least 2 hours. Then, for the cleanest slices, I recommend chilling them in the fridge for another hour before cutting. Use the parchment overhang to lift the slab out of the tin before slicing into 16 squares.