Pre-heat the oven to 350ºF.
Butter an 8-inch square pan. Combine the butter, confectioners’ sugar, flour and salt and press into the bottom of the prepared pan, making a crust. Combine the eggs, granulated sugar, the remaining 2 1/2 tablespoons flour, lemon juice and lemon zest and beat until light in color. Pour over the crust and bake for 25-30 minutes. Let cool until cool to the touch, then dust the top with confectioners’ sugar. Cut into uniform squares.