Preheat oven to 350° F. Lightly grease a square 8x8 baking pan. You can also line the pan with parchment paper if disired. Set the prepared pan aside.
In a large mixing bowl, cream together the butter and brown sugar using a sturdy fork, a pastry blender, or a hand mixer.
In a separate smaller mixing bowl whisk together flour, salt, and baking powder. Add oats to the dry ingredients and stir to combine. Add dry ingredients to the butter mixture and mix to combine.
Press half or slightly more of the oat mixture into the prepared pan. In a small bowl mix together sweetened condensed milk, lemon juice, lemon zest, and lemon extract. Spread evenly on top of the oat mixture in the pan. Top with the remaining oat mixture.
Bake for 20 to 25 minutes or until the top is golden brown. Let cool and refrigerate for 30 minutes or until set. Cut into bars and store in the refrigerator.