Preheat oven to 350 degrees F and line an 8x8 square baking pan with parchment paper. Set aside.
Crush graham crackers finely and mix with brown sugar. Stir in melted butter and place into prepared baking pan. Use the back of the a spoon to press crust into the bottom of the pan. Bake for 10 minutes and let cool.
In a mixing bowl, combine sweetened condensed milk, egg yolks, zest of 2 lemons, ½ cup fresh squeezed lemon juice (juice from about 2 large lemons), vanilla and salt. Whisk together until smooth.
Pour on top of crust and bake for 22-26 minutes or until edges just begin to brown and center is no longer jiggly. When cooking is complete, cool and cover and refrigerate overnight.
When ready to serve, cut into bars and sprinkle with powdered sugar.