Preheat oven to 350. In a medium mixing bowl whisk together flour, rolled oats, salt and baking soda. Stir in sugar and brown sugar and mix until no clumps of sugar remain.
Stir vanilla extract into the melted butter and pour mixture over dry ingredients. Using a spoon stir mixture evenly moistened.
Sprinkle half of the crumb mixture into the bottom of a greased 8x8 inch baking dish and gently press into a even layer.
Bake in preheated oven for 15 minutes. Remove from the oven and sit aside leaving oven heated at 350.
In a mixing bowl whisk together the sweetened condensed milk, lemon zest, lemon juice, egg yolks and lemon extract until well blended. Pour mixture over crumb crust in baking dish and spread into an even layer.
Sprinkle top evenly with the remaining crumb mixture. Bake in 350 oven for 23-26 minutes until lightly golden.
Remove from oven and allow to cool at room temperature once cool cover and refrigerate for 30 minutes to 1 hour then remove and cut into squares. Store in airtight container in refrigerator.