Preheat oven to 350°F (175°C) and line a baking sheet with parchment.
Spread pecan halves in a single layer and toast 3-5 minutes until fragrant. Set aside.
In a large bowl, whisk melted butter with brown sugar and granulated sugar until combined.
Stir in egg, egg yolk, and vanilla until smooth.
In a separate bowl, whisk flour, cornstarch, baking soda, baking powder, and salt.
Gradually add dry ingredients to wet, stirring just until no flour streaks remain.
Coarsely chop toasted pecans and fold into dough along with toffee bits.
Line two baking sheets with parchment. Scoop heaping 1 ½-tablespoon portions of dough 2 inches apart.
Bake at 350°F for 10–12 minutes until edges are set but centers look soft and underdone.
Sprinkle warm cookies with flaky sea salt if desired.
Slide parchment onto a wire rack and let cookies cool completely on the pan (15–20 minutes).