Preheat oven to 375 degrees. Line two cookie sheets with parchment paper.
In large bowl, cream together softened butter and sugars on low speed. Add eggs, sour cream and vanilla. Mix on medium speed for 2 minutes.
In medium bowl, stir together dry ingredients. Slowly add dry ingredients to the creamed mixture and mix just until incorporated. DO NOT OVER MIX.
Stir in chocolate chips and nuts (if using).
Use a 1/4 cup measure or large ice cream scoop and fill to heaping. Scoop dough out and roll into ball. Place 6 balls of dough, 3 inches apart on prepared cookie sheets.
Place cookie sheets on two center oven racks. Bake 16 - 18 minutes, switching cookie sheet from top to bottom and turn each cookie sheet 180 degrees half-way through baking time. (Move top cookie sheet to bottom, move bottom cookie sheet to top AND turn each cookie sheet so the front edge [nearest the oven door] of the sheet is turned to the back of the oven.)
Cool cookies for 2 minutes on sheet; move to cooling rack to cool completely.