1. Preheat oven to 350 degrees F. Line a 13x9x2-inch baking pan with foil, extending foil over the pan edges. Set aside. In a large bowl, stir together oats, flour, brown sugar, and baking soda; set aside. In a small saucepan, stir together the butter and 1/3 cup peanut butter. Heat over low heat until melted and smooth. Add the melted peanut butter mixture to the dry mixture. Stir until well combined. Press mixture evenly into bottom of prepared pan. Bake in the preheated oven for 15 minutes or until golden brown. Place pan on a wire rack.
2. In a large saucepan, combine dark chocolate and 2/3 cup peanut butter. Heat over low heat until melted and smooth, stirring occasionally. Spread chocolate mixture evenly over warm crust. Sprinkle toffee pieces evenly over the chocolate mixture. Cool in the pan on a wire rack for 2 hours or until chocolate sets. Using the edges of the foil, lift the uncut bars out of the pan. Cut into bars. Makes 24 bars.