Pre-heat oven to 350 degrees.
Line 2 baking sheets with parchment paper.
In a medium bowl with an electric mixer on high-speed, cream the butter, brown sugar, and vanilla.
Mix in dry ingredients; just until blended.
Fold in oats, toffee bars, and walnuts OR pecans.
Chill the dough for 20 minutes in the refrigerator.
Drop cookies using ice cream scoop onto the baking sheets lined with parchment paper.
Bake 15 to 18 minutes until the edges are golden but still wet looking in the middle.
Cool on a wire rack.