Preheat the oven to 175C (160C fan). Lightly grease 8 x 8 square baking tin and line it with baking paper, leaving plenty of overhang (it will help with removing the bars later from the tin).
Place the butter and sugar in a mixing bowl and using an electric mixer, beat together for about 2 minutes until combined. Add the flour and mix on low speed until just combined. Transfer the mixture into the baking tin, and press firmly (you can use a back of a spoon or measuring cup too) with your hands. Place in the oven and bake for 15- 18 minutes until pale golden. Remove from the oven and set aside. If the surface of the shortcrust base becomes a bit uneven and 'bubbly' after baking, gently press it with the bottom of a measuring cup or a back of the spoon, while it is still warm, to even the surface out. Reduce the oven temperature to 160C (150C fan).
While the base is baking, prepare the lemon curd. Place the heatproof bowl over a pan of gently simmering water. Add strained lemon juice, lemon zest, sugar and butter. Stir often, until all the butter have melted. Lightly beat the eggs and an extra egg yolk in a jug, then slowly pour it into the mixture, stirring continuously. Keep stirring until the mixture becomes thicker and covers the back of the spoon (about 5-10 minutes). Remove from the heat.
Pour the lemon curd over pre-baked shortcrust base and place in the oven. Bake for 15-20 minutes, until the edges just start to bubble and the middle of the bake still has a gentle wobble to it (like a firm jello). Remove from the oven and allow it to cool completely, then transfer to the fridge for at least a couple of hours, or ideally, overnight.
Once cold and set, remove the bake from the tin by gently lifting it by overhang baking paper. Carefully peel away the baking paper, then slice it into 16 squares, using sharp, large knife. For a clean and precise cut, run the knife under hot water and wipe it clean between each cut. Dust the bars with icing sugar (optional), decorate with some extra lemon slices if desired, and enjoy!
For the detailed instructions and troubleshooting lemon curd, read this post
If the edges of the bake become really bubbly after baking, simply trim the away before slicing the rest of the bake into squares.
Make sure to allow the bake to cool. The filling need time to set and firm up properly. If you try cutting it too early, it will spill and become messy.