Preheat oven to 350 F. Beat butter in a large bowl until smooth. Beat in sugar until well blended. Add flour, one cup at a time. A wooden spoon works best but the final (4th) cup you may need to incorporate by hand mixing. Dough will be very stiff. Spread batter into a 9 x 13″ baking pan, pressing down evenly to all edges. Using the tines of a fork, go around the edges pressing tines against the edge, then with tines, prick multiple scattered vent holes throughout dough.
Bake at for 45 to 60 minutes until golden brown. Upon removal from oven, immediately cut into small 2″ pieces but leave in pan to continue to cool. Store in cookie tin.