In a large bowl (or bowl of electric mixer) cream the butter until fluffy. Gradually add confectioners sugar and mix well. Work in flour until dough comes together and can be easily handled. (It usually takes closer to 5 cups for me).
Turn batter into a 9 x 13 pan and pat down firmly. Prick all over with a fork and bake for 50 to 60 minutes or until edges are golden. Start checking at about 50 minutes.
Notes
Store in an airtight container at room temperature for up to 4 days, or freeze for up to 2 months.