Instructions

1. First, toast the walnuts. Preheat the oven to 375°F and line a small baking sheet with parchment paper. Spread the walnuts out into an even layer on the sheet and bake for 5 to 7 minutes, stirring the nuts halfway through. You’ll know they’re done when they’re lightly golden brown all over and smell toasty. Set aside to cool.
2. Next, brown the butter. Place the butter in a small saucepan and set over medium low heat. Low heat is key to an even, golden browning! Use a silicone spatula to stir and scrape the butter from the bottom and sides of the pan every 10 to 15 seconds. At first, the butter will melt and foam, then over time the foam bubbles will get smaller and the butter will begin to emit a warm, nutty aroma. Continue to cook the butter, occasionally stirring and scraping with your spatula, until the butter begins to take on color and you see small floating brown bits, about 5 to 7 minutes (these are the milk solids in your butter separating and toasting). Once the butter is golden brown, remove it from the heat and allow it to cool in the saucepan for a few minutes.
3. In a large mixing bowl, combine the brown butter, brown sugar, and granulated sugar. Use a silicone spatula to mix together. Add the egg, extra egg yolk, and vanilla and mix well.
4. Add in the flour, baking soda, salt and fold until no streaks of flour remain, being careful not to overmix your batter. The dough should be soft but not sticky, so when you touch it with your finger your finger doesn’t stick to the dough.
5. Fold in the chocolate chips and walnuts until evenly distributed. Do not over mix. Cover the bowl and place it in the fridge to chill for 30 minutes. After 30 minutes, remove the bowl from the fridge and use a 1.5 ounce cookie scoop to scoop out your cookies. Place the cookie dough balls in a sealed container or on a cookie sheet wrapped in plastic wrap and place back in the fridge for at least another 30 minutes, but up to 24 hours!
6. Preheat the oven to 350°F. Line two baking sheets with parchment paper. Place the cookie dough balls on the prepared baking sheets, spacing the dough 2 inches apart. 
7. Bake until the cookies have puffed up and are light golden brown around the edges but still somewhat soft in the middle, 12 to 13 minutes. Remove the baking sheet from the oven, sprinkle with flaky sea salt (if using), and allow the cookies to cool slightly on the baking sheet before serving.