In a large bowl, use an electric mixer to beat together the butter and shortening on medium speed for 30 seconds. Add the brown sugar, baking soda and ¼ teaspoon of salt; beat until light and fluffy. Add the eggs and vanilla; beat well.
Add the flour, mixing at low speed just until combined. Stir in the M&M's and nuts (if desired).
Drop the dough from rounded teaspoons 2 inches apart onto the prepared baking sheets. Bake for 8 to 10 minutes or until golden. Cool the cookies for 1 minute on the baking sheet. Using a wide spatula, transfer the cookies to wire racks to cool completely.
Notes
To store, place in layers separated by waxed paper in an airtight container; cover. Store at room temperature for up to 3 days or freeze for up to 3 months.