For the crust:
- Preheat oven to 350°F/180°C/Gas Mark 4.
- Add 2 cups flour and ½ cup sugar into a medium bowl; mix to combine. Cut in softened butter with a knife or a pastry blender to make wet crumbs.
- Press the prepared crumbs into the bottom of an ungreased 9x13 baking pan and 1 inch up the sides to make a border.
- Bake the crust for 20 minutes.
While baking, make the filling.
For the filling:
- Beat condensed milk with eggs until light and fluffy. Add lemon juice in a thin streak, beat until well-combined, then beat in baking powder, 1 tbsp flour, salt. Fold in grated lemon peel very carefully in order not to spoil the fluffy texture.
Pour the filling over the baked crust, return to oven and bake until the filling is set – about 20 minutes or 5 minutes more.
Remove from oven, let cool in the pan to room temperature, then refrigerate for at least 2 hours.
Cut into serving-size bars, sprinkle with powdered sugar, if desired.