1-Preheat oven to 350°F: Preheat oven to 350°F.
2-Butter and line an 8-inch square baking: 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).
3-Break up the graham crackers: 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.
4-Add this crumble mixture to the prepared: 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.
5-Bake in the preheated oven for 10-12: Bake in the preheated oven for 10-12 minutes, or until lightly browned. Let the crust cool completely in the pan.
6-Make the creamy filling while the crust: 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.
7-When the graham cracker crust has cooled,: 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.