Preheat oven to 350°F (180°C).

In a food processor or using a pastry cutter, mix together the softened butter, flour, lemon zest, sugar, and salt until it creates a soft dough.

Line a 9x9 inch ( 23 x 23 cm) baking pan with parchment paper, then press this mixture firmly into the tray. Bake it for approximately 20-22 minutes until lightly browned and just baked through.

In a separate bowl, whisk together the flour and the sugar for the lemon layer until well combined. Add the eggs, lemon zest, lemon juice, and salt, and whisk until smooth. Pour the mixture of the baked base and place it back in the oven.

Bake for around 20 minutes until the lemon layer has set. Giving the pan a little shake should show no wobble left to the lemon layer.

Once baked, cool the lemon squares to room temperature, then refrigerate for at 1-2 hours before slicing. Powdered sugar can be dusted over the top at the time of serving.