1(14 oz) can sweetened condensed milk (not evaporated)
Preheat oven to 350 degrees. Grease a square 8x8 baking pan and set aside.
In a large mixing bowl, cream together the butter and brown sugar with an electric mixer over medium speed until crumbly.
In a separate smaller mixing bowl whisk together flour, salt, and baking powder. Add oats to the dry ingredients and stir to combine. Add the dry ingredients to the butter mixture and mix well.
Press 3/4 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, then refrigerate for 30 minutes or until set. Cut into bars and store in the refrigerator.