Preheat oven to 350ºF. Prepare a 13x9-inch pan with cooking spray or butter.
In a large bowl, blend brown sugar and butter until well combined and thick, like paste. Add flours, oats, salt and baking powder and mix together with a fork or your fingers combined. It should stick together in small clumps. Press half of oat crumb mixture into the bottom of prepared pan.
In a medium bowl, whisk together condensed milk, lemon juice, lemon zest and vanilla. Spread evenly over layer of pressed oat crumbs.
Sprinkle the remaining oat crumbs over the top of the lemon layer; do not press down this time.. Bake in a 350˚F oven for 20-25 minutes or until golden brown. Cool the pan on a wire rack for 30 minutes and then cut the bars into squares. Refrigerate the bars for several hours before serving. Serve chilled.