1. In a bowl, with a mixer on medium speed, beat butter, granulated sugar, and brown sugar until smooth. Add vanilla and eggs and beat until well blended.
2. Stir in flour, salt, and baking soda until well combined. Stir in butterscotch chips, chocolate chips, and walnuts. Butter a 9- by 13-inch baking pan and spread dough level in pan.
3. Bake in a 350° regular or convection oven until top is browned and a wooden skewer inserted in center comes out with moist crumbs attached, about 30 minutes. Cool on a rack for 15 minutes, then cut into 24 rectangles. Serve warm or cool.
Nutritional analysis per bar.