Preheat the oven to 350°F.
Butter and line an 8-inch square baking pan with parchment paper, leaving excess paper hanging on two sides for easy removal.
Break graham crackers into pieces and pulse them in a food processor until crumbled.
Add sugar and melted butter to the crumbs and pulse until combined.
Press the crumb mixture firmly into the bottom and 1 inch up the sides of the prepared pan to form the crust.
Bake the crust for 10-12 minutes, or until lightly browned, then cool completely in the pan.
In a bowl, whisk together the sweetened condensed milk and egg yolks until smooth.
Add lemon juice and whisk until well combined to make the filling.
Pour the filling over the cooled crust and bake for 15-20 minutes, until the filling is set and crust is browned.
Cool the lemon squares completely in the pan on a rack; refrigerate if desired to firm up.
Lift the squares out using the parchment paper and cut into 9 or 16 pieces as preferred.