In a medium saucepan, melt butter over medium heat. Stir frequently as it foams and browns—look for amber color and a nutty aroma. Remove from heat immediately and transfer to a large bowl to cool for 10 minutes.

Whisk brown butter with both sugars until glossy. Add eggs and vanilla, instant espresso and whisk until well combined and slightly thickened.

In a separate bowl, whisk together flour, salt, baking soda, and baking powder. Gradually fold into the wet mixture using a spatula or wooden spoon.‍

Stir in toffee bits and chocolate chips until evenly distributed.

Cover and chill dough in the fridge for at least 30 minutes (up to 48 hours for richer flavor).

Preheat oven to 350°F (175°C). Line baking sheets with parchment paper. Scoop ~2-tbsp dough balls and place 2 inches apart. Press extra chocolate on top if desired.

Bake for 10–11 minutes, until edges are golden and centers look just slightly underbaked.‍

Immediately sprinkle with flaky sea salt. Let cookies cool on the baking sheet for 5 minutes, then transfer to a wire rack to completely cool for at least 20 mins.