Preheat oven to 350 degrees. Spray a 9x9 inch baking pan with nonstick spray. Combine butter, brown sugar, flour, baking powder and salt with the oats. Put 2/3 of the mixture in the bottom of your baking pan. Set remainder of the oat mixture aside for a later step.
In a medium bowl, whisk together sweetened condensed milk, lime juice and zest. Pour over the top of the oat mixture in the pan. Crumble all the remaining oat mixture over the top of the lime filling and bake for 30 minutes until golden brown.
Allow to cool completely before cutting. Keep in the fridge if you prefer them to be firmer.