Preheat oven to 350°F and line a 9x13" baking pan with parchment paper.
Add the butter, sugar and lemon zest to a large bowl; mix until light and fluffy.
Add the salt, flour and oats and stir until combined.
Press about half of the mixture into the bottom of the pan and bake for 12 minutes. While the crust is baking, make the cream cheese filling.
Place cream cheese in a bowl and mix until nice and smooth.
Add the sweetened condensed milk, lemon juice, lemon zest, yolk and flour and mix until well-combined.
Pour over the baked crust and smooth out with the back of a spoon.
Sprinkle the remaining crumb topping over the cream cheese mixture and bake for 20-25 minutes or until they're a light golden brown around the edges.
Let cool for 2+ hours in the fridge, then cut into bars and enjoy!