Heat up the oven to 175°C (350°F)
Softened up the butter in the microwave for around 30 sec then mix it together with the sugar, oats, flour and baking powder
Use a baking pan of around 20*20cm (8*8 inches) and line it with parchement paper. Add 2/3 of the oat mix to the pan and press it out evenly
Take another bowl and mix the condensed milk with lemon juice and the zest
Spread out evenly in the pan and finish by adding the remaining oat mixture on top
Bake for 20 min then let it cool before you cut it in squares
You can store it either in the fridge or in the freezer. I actually prefer to eat it really cold straight from the freezer