For the crust: Cream the butter and sugar together until light and fluffy. Add the egg and mix well. Stir in the flour and salt and knead until a smooth dough is formed. Shape into a ball, cover with plastic wrap and refrigerate for 1 hour.
Then preheat the oven to 160°C (approximately 350°F). Roll the dough out onto a floured surface and place into the baking pan. Prick several times with a fork and bake for 15 minutes. Remove from oven and allow to cool slightly.
For the filling: Whisk the eggs with the sugar until light and fluffy. Add the salt, cornstarch, lemon juice and lemon zest. Mix well.
Pour the filling into the crust and bake for 25-35 minutes, until lightly golden.
Remove from the oven and allow to cool. Cut into small squares and serve dusted with powdered sugar.