Begin by setting the oven temperature to 350 degrees Fahrenheit (175 degrees Celsius). This will ensure the environment is prepared for the baking process.

Prepare an 8-inch square baking pan by applying a thin layer of butter to the surface to prevent sticking. Follow this by lining the pan with parchment paper, allowing the edges to overhang to facilitate the removal of the lemon squares after baking.

Proceed to the crust preparation by breaking the graham crackers into smaller pieces. Place these pieces into a food processor and pulse until they reach a fine, crumbled texture. To this, add the granulated sugar and the melted unsalted butter, pulsing the mixture again until all components are thoroughly combined.

Transfer the crust mixture into the previously prepared pan. Firmly press the mixture down into the bottom of the pan and slightly up the sides, creating a packed and even crust layer. This will serve as the foundation for the filling.

Place the crust in the preheated oven and bake for a duration of 10 to 12 minutes, or until a light golden brown hue is achieved. Upon removal from the oven, allow the crust to cool entirely within the pan.

While the crust is cooling, commence the preparation of the lemon filling. In a suitable mixing bowl, whisk together the egg yolks and sweetened condensed milk until a smooth and homogenous mixture is obtained. For optimal results, one may employ an electric mixer or a whisk. Then, incorporate the fresh lemon juice into the mixture, continuing to whisk until the ingredients are fully integrated.

Once the crust has adequately cooled, pour the prepared lemon filling over it, ensuring an even distribution. Return the pan to the oven and bake for an additional 15 to 20 minutes, monitoring until the filling sets and the crust adopts a deeper golden-brown coloration.

After baking, it is imperative to allow the lemon squares to cool completely within the pan, placed on a cooling rack. For a firmer texture, one may opt to refrigerate the squares after they reach room temperature.

To serve, remove the lemon squares from the pan by lifting the overhanging edges of the parchment paper. Transfer them to a cutting board and proceed to slice the dessert into squares, according to your size preference—either 16 small squares or 9 larger ones. This dessert is now ready to be served and can be stored as detailed in previous storage instructions.