Preheat the oven to 180C/350F/Gas 4. Line an 8x8-inch square baking pan with parchment paper or aluminium foil, and set aside.
Whisk together the flour, baking powder, salt, sugar, and oats. Add the melted butter, and stir until the mixture is moist and crumbly. Press half of the mixture into the prepared pan, and bake for 10 - 15 minutes until lightly golden.
Stir together the condensed milk, lemon juice, and zest. Pour over the baked base, and sprinkle with the remaining crumb mixture. Gently press the crumbs down. Bake for 30 - 35 minutes, or until lightly golden.
Allow to cool completely in the pan on a wire rack. Refrigerate for at least 1 hour before slicing into bars.
For the Glaze
Whisk together the icing sugar, lemon juice, and milk until combined. Add more milk if the glaze is too thick, or more icing sugar if too runny. Drizzle the glaze over the cooled bars.
Notes
Bars will stay fresh in an airtight container in the fridge for up to 5 days. They also freeze well for up to 3 months. Thaw overnight in the fridge before serving.