Instructions:

Preheat oven to 350°F.
Butter and line an 8-inch square baking pan with parchment paper (leave some paper hanging on two sides, it will help to remove the creamy lemon squares).
Break up the graham crackers. Then, add them to a food processor. Pulse until crumbled. Add sugar and melted butter. Pulse one more time until combined.
Add this crumble mixture to the prepared pan. Then, press this mixture firmly into the bottom and 1 inch up the sides of the pan to create a crust.
Bake in the preheated oven for 10-12 minutes, or until lightly browned. Let the crust cool completely in the pan.
Make the creamy filling while the crust is cooling. Add condensed milk and egg yolk to a bowl. Whisk until smooth. You can use an electric mixer or use a whisker. Then, add fresh lemon juice and whisk until well combined.
When the graham cracker crust has cooled, pour this creamy lemon filling into it. Bake in the preheated oven for 15-20 minutes, or until the filling is set and the crust is browned. Let the creamy square completely cool in the pan on a cooling rack. You can also refrigerate the lemon squares after cooling to room temperature; it makes the creamy lemon squares firmer. Then, remove the lemon square from the pan using the hanging parchment paper, and place it on a cutting board. Cut the lemon square into 16 mini squares or 9 bigger squares.