Preheat oven to 325°F (163°C). Line an 8x8 or 9x9 inch baking pan with parchment paper, leaving a few inches of overhang on both sides, and lightly spray with non-stick cooking spray.
Combine graham cracker crumbs, sugar, and melted butter in a medium bowl, stirring until the mixture resembles wet sand. Press firmly into an even layer across the bottom of the prepared pan.
Bake the crust for 8 to 10 minutes until lightly golden and set. Remove from the oven and set aside to cool slightly while you prepare the filling; keep the oven on.
Zest 2 lemons to get 1 tablespoon of zest, then juice enough lemons to measure ¼ cup of fresh lemon juice. Strain juice to remove any seeds.
Beat the softened cream cheese on medium speed for about 2 minutes until completely smooth and lump-free, scraping down the sides of the bowl as needed.
Mix in the granulated sugar on medium speed for 1 minute until creamy, then add the eggs one at a time on low speed, mixing just until each is incorporated.
Add the fresh lemon juice, lemon zest, vanilla extract, and sour cream, mixing on low speed just until smooth and fully combined. Scrape down the sides once more.
Pour the filling over the pre-baked crust, spread evenly with a spatula, and gently tap the pan on the counter to release air bubbles. Bake at 325°F for 30 to 35 minutes, until edges are set and the center has a very slight jiggle.
Remove from the oven and place on a wire cooling rack. Allow the bars to cool at room temperature for at least 1 full hour before refrigerating.
Cover the pan tightly with plastic wrap and refrigerate for at least 2 hours, or overnight for the best texture and flavor.
Lift the bars out using the parchment overhang and transfer to a cutting board. Run a hot, dry knife through the bars, wiping clean between each cut, then slice into 16 squares. Dust with powdered sugar and garnish as desired before serving.