Preheat the oven to 180°C/350°F and line a 20cm x 20cm (8in x 8in) baking pan with parchment paper.
In the bowl of a food processor, blend the butter, flour, sugar, lemon zest and salt until the mixture just comes together.
Press the mixture into the base of the prepared pan.
Place in the oven and bake for 15-20 minutes until golden brown. Remove from the oven and allow to cool.
Whisk together the condensed milk, fresh lemon juice, lemon zest, egg yolks and pureed raspberries (you can strain the puree if you'd prefer to remove the seeds) until smooth.
Pour onto the base then place back into the oven and allow to bake for 20-25 minutes until the edge is set but the center is still slightly jiggly.
Remove from the oven and allow to cool completely in the pan (ideally overnight in the fridge) until fully set before slicing.