Preheat oven to 350˚F. Grease and line a 9×9 inch pan with foil or parchment paper.
Using a food processor, pulse the shortbread cookies until you have an even, fine crumb mixture. Transfer to a large bowl and stir in the melted butter, sugar, zest and salt. Press this mixture into the lined pan, creating a level shortbread base. Bake in the oven for 10 minutes and then set aside.
While the base is baking, make your filling. Using an electric hand mixer, beat together the eggs, juice and zest. Add in the rest of the ingredients, except the icing sugar, and continue to beat for three minutes. Pour over top of the base when it comes out of the oven and return to the oven to continue baking for 25-30 minutes, or until the center looks set.
Leave the lemon bars to cool in the pan. Once the pan has completely cooled down, place it in the fridge and chill overnight. When ready to serve, dust the lemon bars with icing sugar.
If you do not have a food processor, simply place the cookies in a sturdy ziploc bag and crush with a rolling pin until you have a fine crumb.
Chilling overnight can be skipped but I do find it helps the bars set up nicely before being cut.