Preheat the oven to 350°F(177C°). Grease 8 inch (20cm) square oven-proof dish and line the bottom and sides with parchment paper, about 2 inches(5 cm) of overhang on each side.
Crust: Mix the flour and sugar in a medium bowl. Add in the cubed butters and cut butter into flour mixture with a fork. Alternatively, you can rub the butter and flour mixture with your fingertips. The mixture will look like breadcrumbs.
Spread the mixture into the oven proof dish with your hand. You can use the back of a spoon to spread it evenly. You should lightly flour the spoon to avoid it sticking to the flour-butter mixture. Bake for almost 25-28 minutes until it is golden brown. For a soft crust: Bake for 20-23 minutes or until it lightly changes color.
Filling: Mix the lemon zest and sugar in a bowl with a fork or rub with your fingertips to increase the lemon flavor. Add in the eggs and whisk for almost 30 seconds or until combined well. Then add the cooled melted butter, lemon juice and mix. Add the flour and mix until incorporated.
After 25 minutes (or almost 20 minutes for a soft crust), remove the dish from the oven and add the filling. Bake for 12-15 minutes until the filling is set but still soft. Remove the dish from the oven and let it cool at room temperature for almost 1,5 hours. When it is cool enough, lift the paper from the sides. Cut into 9 or 16 squares. Dust with powdered sugar.