1. Melt butter in a skillet over medium heat. Add chopped pecans and toast for 4-5 minutes until lightly toasted. Set aside to cool.
2. In a separate saucepan, melt 2 sticks of unsalted butter over medium heat and cook until golden brown with a nutty aroma. Pour into a large mixing bowl and let cool slightly.
3. In another bowl, whisk together the flour, cornstarch, salt, cinnamon, and baking soda.
4. Add the dark brown sugar and granulated sugar to the browned butter, stirring until combined. Mix in vanilla extract. Add eggs one at a time, mixing thoroughly after each addition.
5. Gradually fold the dry ingredients into the wet mixture until just combined. Stir in the toasted pecans and optional toffee bits.
6. Cover the dough and refrigerate for at least 4 hours or overnight to develop flavor and texture.
7. Preheat oven to 375°F (190°C). Line baking sheets with parchment paper. Form dough into 3-tablespoon balls and place them 3 inches apart on prepared sheets. Optionally, press a pecan half on top of each cookie.
8. Bake cookies for 10-11 minutes. Remove from oven when edges are set but centers remain soft. Sprinkle with flaky sea salt if desired.
9. Cool cookies on the baking sheet for 15 minutes, then transfer to a cooling rack to cool completely.