Preheat oven to 350 degrees and spray an 8x11 inch pan (see Note) with cooking spray.
Mix butter and brown sugar until well combined. Stir together flour, oats, baking powder, and salt. Add to the butter/sugar mixture and mix to combine. Press half of oat crumb mixture into the bottom of the pan.
Mix together condensed milk, lemon juice, and lemon zest. Spread onto the bottom layer of the crumb mixture. Take the other half of the crumb mixture mix and squeeze together with clean hands. Crumble on top of the lemon layer.
Bake for 20 to 25 minutes, or until top is golden brown.
Allow pan to sit on counter for 30 minutes after baking. Cut into squares and refrigerate for a couple of hours or until cool.
Serve cool. Makes around one dozen. You can top with strawberries and whipped cream if desired.
Notes
An 8x11 dish isn't one that everyone has. You can use a 9x13 baking dish but they will be really thin. My suggestion is to roll up foil and place it on one end of a 9x13 to make a barrier the size of an 8x11 if you don't have one.Source: The Pioneer Woman