Preheat your oven to 350°F (175°C). Line a 8x8-inch baking pan with parchment paper, leaving an overhang on the sides to easily lift the bars out after baking.
In a food processor, pulse the graham crackers until they form fine crumbs. Add the melted butter and brown sugar, and pulse until well combined. Transfer the crumb mixture to the prepared baking pan.
12 graham crackers, 5 Tablespoons salted butter, melted, 1/4 cup brown sugar
Using your hands and the bottom of a flat cup, press the mixture into an even layer on the bottom of the pan, but do not press it up the sides.
Bake for 12 minutes or until golden brown. Remove from the oven and let the crust cool on a cooling rack while you prepare the filling.
In a large bowl, whisk together the lemon zest, lemon juice, sweetened condensed milk, yogurt, and vanilla extract until smooth and well combined.
2 large lemons, zested, 3/4 cup freshly squeezed lemon juice, 2 14- ounce cans sweetened condensed milk, 1 cup plain unsweetened Greek yogurt*, 1/2 teaspoon vanilla extract
Pour the filling over the semi cooled crust.
Bake for 18-20 minutes. The filling will still be wobbly, but will fully set as the bars cool. Allow the bars to cool on a cooling rack for 1 hour at room temperature. Transfer the pan to the refrigerator and chill for at least 4 hours or overnight, until fully set.
Once completely cooled, use the parchment paper overhang to lift the bars out of the pan. Place on a cutting board. Using a sharp knife, slice into 16 bars. Serve with whipped cream, if desired.
whipped cream, optional for serving