• Combine the cold butter and powdered sugar in the large bowl of a stand mixer and beat until smooth and creamy, scraping down the sides of the bowl as needed.
  
• Mix in the vanilla and salt, then slowly add the flour on low speed, mixing just until the dough comes together.  
• Stir in the toffee bits and chopped pecans, then divide the dough into two even-sized portions and shaped into logs that are each 6 ½-inches long and approximately 1 ½ to 1 ¾-inches in diameter.  I find it's easiest to squeeze the dough tightly to compress and make sure there aren't any air pockets in the center, then roll it out on a lightly floured surface until it's the right size.
• Wrap each log tightly in plastic wrap and refrigerate for 1 hour until chilled.
• Heat oven to 350 degrees before removing the shortbread dough from the fridge.  Line a baking sheet with parchment paper, then slice each log into 14-15 slices, each slightly less than ½-inch wide.  Arrange the sliced shortbread dough on the prepared baking sheet, then bake for 12-16 minutes, until just barely turning golden brown around the edges.  
• Remove from oven and cool completely on a wire rack.  

Chocolate Dipping

• If you choose to dip the cooled toffee pecan shortbread cookies in chocolate, melt the chocolate in a microwave-safe bowl for 20 seconds at a time on the defrost setting, stirring between each burst of heat, until completely melted.
• Dip half of each cookie in the chocolate, then shake off the extra and lay flat on a parchment-lined baking sheet.  Sprinkle with additional toffee bits or chopped pecans before the chocolate has a chance to set, then allow the chocolate to set completely for about an hour at room temperature or in the refrigerator for quicker results.  
• Store in an airtight container for up to 5 days at room temperature.  These cookies also freeze very well.