Cup of Yum
1. 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.
2. Mix in the vanilla and salt, then slowly add the flour on low speed, mixing just until the dough comes together.  
3. 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 1/2-inches long and approximately 1 1/2 to 1 3/4-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.
4. Wrap each log tightly in plastic wrap and refrigerate for 1 hour until chilled.
5. Heat oven to 350°F 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 1/2-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.  
6. Remove from oven and cool completely on a wire rack.  

Chocolate Dipping

1. 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.
2. 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.  
3. Store in an airtight container for up to 5 days at room temperature. These cookies also freeze very well.