Start by preheating the oven to 350℉ and preparing a square (9x9) baking pan by lining it with two sheets of parchment paper that criss-cross. Make sure there is enough excess parchment paper hanging over the sides of the pan (this will make removing the filling from the pan easier later.)
In a food processor, grind the graham crackers into a fine crumb. Mix in melted butter until all the graham is moist.
Dump the crumbs into the baking pan and pack down to form the crust. (Use the back of a spoon, measuring cup, or spatula)
Bake the crust for 8-10 minutes, then lower the oven temperature to 325℉
While the crust is baking, zest and juice the lemons.
Mix the zest, lemon juice, vanilla, sweetened condensed milk, and yogurt with a hand whisk until combined. Taste and adjust as needed.
If you prefer a bolder lemon flavor, you can add more lemon zest or add more lemon juice.
Add the egg yolks and whisk until combined.
Transfer the filling to the pan on top of the baked crust. Shake and tap the pan on your counter to pop any air bubbles.
Bake for 16-18 minutes at 325℉ until it looks mostly set around the edges, but slightly jiggly in the center. As it chills, it will set.
Refrigerate overnight
Once the bars are completely set, you can prepare to add the whipped topping. Mix the heavy whipping cream and sugar on a low setting until stiff peaks form.
Spread the whipped topping over the lemon filling in the pan.
Using the excess parchment paper, carefully lift the bars from the pan onto a cutting board or serving platter.
Zest a lemon over the whipped topping for garnish. Slice into squares, serve, and enjoy!