Preheat oven to 350F (175C) and line a cookie sheet with parchment paper.
Spread pecan halves evenly over the cookie sheet and bake on 350F (175C) for 3-5 minutes.
Remove from oven (do not turn off your oven) and set aside while you prepare your cookie dough.
In a large bowl, combine melted butter and sugars. Stir well.
Add eggs, egg yolk, and vanilla extract and stir until well-combined.
In a separate, medium-sized bowl, whisk together flour, cornstarch, baking soda, baking powder, and salt.
Gradually add flour mixture to wet ingredients until completely combined.
Add chopped pecans and toffee bits (if using) and stir well.
Line cookie sheets with parchment paper and drop cookie dough by heaping 1½ Tablespoon onto the prepared cookie sheet, spacing at least 2" apart.
Transfer to 350F oven and bake for 10-12 minutes. Cookies should still look soft/slightly under-done in the centers. Allow to cool completely on the baking sheet and they will finish baking through as they cool but will stay nice and soft!