Preheat oven to 350 degrees. Place rack in center of oven. Line a 9x13 pan with foil, leaving an overhang on the ends for easy removal. Lightly spray the bottom and sides with non-stick cooking spray. Set aside.

In a large bowl combine flour, oats, sugars, cinnamon, baking powder and salt. Add the melted butter and stir to combine. Mixture will be crumbly. Set 2 cups of the mixture aside to reserve for topping. Press remaining mixture into bottom of 9x13 pan. Try to get crust as even as possible. Bake for 12 minutes.

In a medium bowl whisk together sweetened condensed milk, lemon juice, lemon zest, egg yolk, and vanilla.

Remove crust from the oven and sprinkle with the blueberries. Drop spoonfuls of the lemon mixture over the blueberries and spread with the back of a spoon. Don't worry it won't be perfectly even. Bake for 8 minutes. Sprinkle reserved topping over the lemon mixture and bake for an additional 30 minutes until lightly browned and bubbly.

Let the bars cool on a wire rack for about 45 minutes. Lift out of the pan by holding the tinfoil overhang and let cool on wire rack completely. Peel back the tinfoil and cut into bars. You can store the bars at room temperature for an hour or two then refrigerate.