Grease an 8-inch square baking pan with butter-flavored cooking spray and preheat the oven to 350 F.
Crumble the graham cookies in a food processor. Add the melted butter, sugar, and salt, and pulse until well combined.
Press the cookie mixture into the bottom of the baking pan using the back of a measuring cup or spoon until well packed and even. Bake the crust for 10 mins. Let it cool down completely (20-30 mins).
While the crust is cooling, place the condensed milk, egg yolks, lemon juice, and lemon zest in a bowl and using a hand-held or stand mixer, beat all ingredients until smooth.
Pour this mixture into the baking pan and over the cooled cookie crust and bake for 10-12 mins until set.
Remove from the oven and let cool completely before placing the pan in the fridge for at least one hour before cutting and serving the squares.