1. Brown the butter: Melt butter in a light saucepan over medium heat. Swirl often until golden-brown bits form and aroma is nutty (~6–8 mins). Pour into a heatproof bowl; chill 1 hour until semi-solid (like room-temp butter).
2. Preheat oven to 350°F (175°C). Line baking sheets with parchment.
3. In a stand mixer, beat browned butter and both sugars until light and fluffy (~3 mins). Add egg, yolk, and vanilla; mix until glossy.
4. Whisk flour, cornstarch, baking soda, and salt. Gradually mix into wet ingredients just until combined. Fold in toffee bits.
5. Chill dough 30 mins (prevents overspreading and deepens flavor).
6. Scoop 3-tbsp mounds (use a #20 scoop). Place 3″ apart on sheets.
7. Bake 11–12 mins—edges crisp and golden, centers still soft and puffed.
8. Immediately sprinkle with flaky sea salt. Cool on sheet 10 mins (centers set slowly, staying chewy).

PREP TIME & NUTRITION :
Prep Time: 20 mins (+1 hr 30 mins chill), Cook Time: 12 mins, Total Time: 2 hrs 2 mins, Yield: 16 cookies
Calories: 240, Net Carbs: 28g, Fats: 14g, Protein: 3g
✅ Lower-sugar option: Use monk fruit brown sugar blend + sugar-free toffee (e.g., ChocZero toffee bits) — net carbs drop to ~6g/cookie.