Step 1: Preheat your oven to 350°F (175°C). Line an 8x8 inch baking pan with parchment paper, leaving an overhang on two sides for easy removal.
Step 2: For the Crust: In a medium bowl, combine the graham cracker crumbs and 1/4 cup granulated sugar. Pour in the melted butter and mix until all crumbs are moistened. Press the mixture firmly into the bottom of the prepared baking pan. Bake for 8-10 minutes, then remove from oven and let cool slightly.
Step 3: For the Cream Cheese Layer: In a mixing bowl, beat the softened cream cheese until smooth. Add the powdered sugar and vanilla extract, beating until light and fluffy. Spread this mixture evenly over the cooled graham cracker crust.
Step 4: Bake for another 15-18 minutes, or until the cream cheese layer is just set. Remove from oven and let cool completely on a wire rack.
Step 5: For the Lemon Topping: In a small saucepan, whisk together 1 cup granulated sugar and cornstarch. Gradually whisk in the lemon juice and egg yolks until smooth. Cook over medium-low heat, whisking constantly, until the mixture thickens and comes to a boil. Boil gently for 1 minute, still whisking.
Step 6: Remove from heat and stir in the butter pieces and lemon zest until the butter is completely melted and incorporated. Pour the hot lemon topping evenly over the cooled cream cheese layer.
Step 7: Refrigerate the bars for at least 3-4 hours, or preferably overnight, until thoroughly chilled and set.
Step 8: Once chilled, use the parchment paper overhangs to lift the bars out of the pan. Cut into 12 squares or rectangles. Serve chilled and enjoy!