1. Brown the butter: Melt butter in a light-colored saucepan over medium heat. Swirl often until it foams, turns golden brown, and smells nutty (~5–7 min). Pour into a heatproof bowl (include browned bits!). Cool 20–30 minutes until just warm.
2. Prep: Preheat oven to 350°F (175°C). Line baking sheets with parchment paper.
3. Mix wet ingredients: Whisk cooled browned butter, brown sugar, and granulated sugar until smooth. Add egg, yolk, and vanilla; mix well.
4. Add dry ingredients: Stir in flour, baking soda, and salt until just combined. Fold in toffee bits.
5. Chill dough 30 minutes (helps prevent overspreading).
6. Scoop: Roll into 2-tbsp balls. Place 3″ apart on sheets.
7. Bake 10–12 minutes, until edges are golden but centers look soft.
8. Finish: Immediately sprinkle with flaky sea salt. Cool on pan 5 minutes before transferring to a rack.

💡 Pro Tips:
– Don’t skip browning the butter—it’s the flavor secret!
– Use chopped toffee bars for bigger, gooier pockets (vs. pre-crumbled bits).
– Store in an airtight container up to 5 days—they stay chewy!

Prep Time & Nutrition (per cookie, makes 16):