First of all, Pre-heat oven to 350 degrees.
In addition, Line 2 baking sheets with parchment paper.
Also, In a medium bowl with an electric mixer on high-speed, cream the butter, brown sugar, egg, and vanilla.
Mix in dry ingredients; just until blended.
Next, Fold in oats, toffee bars, and walnuts OR pecans.
Finally, drop cookies using a regular ice cream scoop onto the baking sheets lined with parchment paper.
Next, Chill the unbaked cookies for 20 minutes in the refrigerator.
For your information, you can also freeze them at this point. Once the cookies are frozen, put them into a ziplock freezer bag.
Bake 15 minutes until the edges are golden but still wet looking in the middle.
The final step, Cool on a wire rack.
For best results, use an electric mixer and cream the butter and shortening.
Slowly add the powder sugar, vanilla, and milk.