Preheat oven to 350°F and lightly grease an 9x9-inch baking dish; set aside.
Combine the flour, oats, salt and baking soda in a medium sized mixing bowl. Using a wooden spoon, stir in the sugars until thoroughly combined. Add the vanilla and melted butter and mix until the dry ingredients are just moistened. Gently press a little less than ¾th of the mixture into the bottom of the prepared pan.
Bake for 12-15 minutes then remove from the oven and set aside, leaving the oven on at 350 degrees.
For the lemon filling:
Whisk together the condensed milk, lemon zest and juice, egg yolks and vanilla in a medium bowl until smooth. Pour into the baking pan over the baked crust. Spread evenly. Crumble the remaining oatmeal crust evenly over top.
Bake at 350 for 20-25 minutes, until light golden.
Remove and let set to cool at room temperature (about 1 hour). Refrigerate for 1 more hour and cut into 16 squares once cooled, or serve warm with ice cream. Store covered in the fridge.
Video
Notes
Serve cold, room temperature, or warm with ice cream.
This recipe can be made in an 8x8-inch pan for thicker bars. I prefer the 9x9 but have made it countless times in the 8x8, too. Baking time may vary.
This recipe is tested with a metal pan. A glass pan may vary in baking time.