Preheat oven to 350 degrees F. Set aside 2 tablespoons of the butter. In a large mixing bowl beat the remaining butter with an electric mixer on medium to high speed for 30 seconds. Add the brown sugar and baking soda. Beat until combined, scraping sides of bowl occasionally. Beat in eggs and 2 teaspoons vanilla until combined. Beat in as much of the flour as you can with the mixer. Stir in any remaining flour. Stir in the rolled oats.

For filling, in a medium saucepan combine the reserved 2 tablespoons butter, the chocolate pieces, and sweetened condensed milk. Cook over low heat until chocolate melts, stirring occasionally. Remove from heat. Stir in the nuts and 2 teaspoons vanilla.

Press two-thirds (about 3 1/3 cups) of the rolled oats mixture into the bottom of an ungreased 15101-inch baking pan. Spread filling evenly over the oats mixture. Dot remaining rolled oats mixture on filling (see photo, below).

Bake for about 25 minutes or until top is lightly browned (chocolate filling will still look moist). Cool pan on a wire rack. Cut into bars. Makes 60 bars.

Prepare as above, except substitute 1/2 cup peanut butter for the 2 tablespoons butter when making the chocolate filling and substitute peanuts for the walnuts or pecans.

Prepare as above, except reduce the all-purpose flour to 1 1/2 cups and add 1 cup whole wheat flour.