In a small bowl, stir together the graham cracker crumbs, brown sugar, and melted butter until completely combined.
Press the crumb mixture into an 8-inch or 9-inch square baking dish that has been lined with two sheets of parchment paper (with the edges of the parchment overhanging the sides of the pan). The parchment will make it easy to lift the bars out of the pan before slicing. Place the crust in the refrigerator while you prepare the filling.
PREPARE THE FILLING
In a large bowl, use an electric hand mixer or a stand mixer to beat together the cream cheese and sugar until smooth and creamy. Use a spatula to scrape down the sides of the bowl, as necessary.
Mix in the Greek yogurt, vanilla extract, lemon zest, and lemon juice just until combined. Gradually add the heavy cream (about ¼ cup at a time), whipping it into the cream cheese mixture after each addition.
Pour the batter over the graham cracker crust. Spread into an even layer (an offset spatula makes this task easy!). Cover and chill in the refrigerator for at least 8 hours or overnight, until fully set.
Carefully lift the bars out of the pan by holding onto the edges of the parchment paper. Slice the bars into squares, wiping the knife clean after each slice. Top with whipped cream, Cool Whip, lemon slices, or fresh berries if desired.
Notes
I use a box of graham cracker crumbs for convenience, but you can crush your own graham crackers in a food processor if you prefer.
Use the flat bottom of a measuring cup to firmly press the crust into an even layer in the pan. This compact crust will hold together better.
Start with softened blocks of cream cheese. It’s easier to mix the cream cheese into a smooth filling when the cream cheese is at room temperature. Do not substitute with tubs of spreadable cream cheese.
Whip the cream into the filling after each addition. This will take at least 30 seconds or so per addition, which helps the cream set up, giving the filling a light, airy texture. Do not substitute with half-and-half or lower-fat alternatives.
When zesting the lemon, be sure that you’re only scraping off the outer layer of the peel (this is where the essential oils and delicious flavor can be found). Be careful not to grate any of the white pith from the fruit, because that is bitter and definitely not what you want in your bars.
This recipe yields 9 large squares. They're very rich, so you might prefer to slice them into 16 smaller bars.