Preheat the oven to 350 degrees F. Line a 8x8 baking dish with parchment paper so that there's about 1-inch overhang - this makes it easier to remove the bars from the pan once they're finished.

Add graham crackers and brown sugar to a food processor and blend until uniform crumbs form. Add melted butter and blend until the texture resembles wet sand.

Add the graham cracker mixture to the baking pan and press firmly into an even layer. Bake in the oven for 12 minutes, then set aside to cool. (leave the oven on, we will need to bake the filling too)

While the crust is baking, make the lemon filling. Add egg yolks into a bowl and gently whisk until combined, about 10 seconds, then add sweetened condensed milk and vanilla extract, then whisk to combine, about 10 seconds. Add lemon zest.

Cut the lemons in half and squeeze out the juice. You will need a full cup of freshly squeezed lemon juice.

Pour lemon juice into the bowl along with sour cream and heavy cream. Whisk to combine, about 20 seconds, then pour the filling on top of the cooled graham cracker crust. Bake for 22-26 minutes, until the edges are set and the center is slightly jiggly, akin to jello.

Allow the bars to cool for about 30 minutes, then place into the refrigerator to set for at least 6 hours, ideally overnight (uncovered).

Just before serving, make the lemon whipped cream. Combine cold heavy cream and powdered sugar in a bowl. Use an electric mixer to mix until stiff peaks form. Then add the lemon juice and lemon zest and mix until just combined. Add on top of the lemon bars.