Combine flour and salt in a large mixing bowl; cut in butter until mixture resembles coarse meal. Dissolve soda in milk; add to flour mixture, stirring well.
Turn dough out onto a floured surface. Pound dough with a wooden mallet or rolling pin for 30 minutes or until surface is covered with air bubbles, folding dough over frequently.
Roll dough to 1/8-inch thickness, and cut into 2-inch squares. Place on well-greased baking sheets in even rows, and prick tops of squares with tines of fork. Bake at 375° for 20 minutes or until lightly browned.