Preheat the oven to 350°F. Grind the graham crackers into a fine crumb using a blender or food processor. In a large bowl, mix the melted butter and graham cracker crumbs until well combined. When pressed together between your fingers, they should stay together.
Line an 8-inch square baking pan with parchment or cooking spray. Press the crust into the pan and slightly up the sides. Use the back of a small measuring cup to ensure that it's firmly packed. If the crust isn't packed firmly to the edges, the filling can seep under the crust.
In a large bowl, whisk together the lemon juice, lemon zest, eggs, sweetener, and flour, until just combined.
Bake the crust for 8-10 minutes until browned and firm.
While the crust is still hot, pour the lemon mixture onto the crust.
Place the dish into the oven and bake for 25-30 minutes until it only slightly jiggles in the center.
Let the bars cool on the counter for 1 hour then place into the refrigerator to cool for 2 hours until fully chilled. Then, dust with powdered sugar before slicing and serving. Store leftovers in an airtight container in the refrigerator for up to four days.