First, Brown the Butter. Melt the butter in a light-colored saucepan over medium heat, whisking constantly until the butter begins to foam. (Watch carefully when the butter begins to foam, as it will turn golden and brown quickly.) When the butter under the foam is golden (like honey) and smells sweet, remove the pan from the heat and transfer to the bowl of your stand mixer or a mixing bowl to cool to room temperature.
Cool, Then Add Sugars. When the butter has cooled, add the sugar and brown sugar. Mix 2-3 minutes or until well combined and the mixture looks like wet sand.
Finish Wet Ingredients. Add eggs + egg yolk one at a time, blending well after each addition. Then, Add the vanilla and mix to combine.
Add Dry Ingredients, Then Mix-Ins. Add baking soda, baking powder, salt, and half of the flour. Mix for a few seconds (it won't be fully combined), then add the rest of the flour, along with the chocolate and toffee bits. Mix until just combined.
Scoop & Chill The Dough. Use a large cookie scoop (3 Tablespoon size) to scoop cookie dough balls. Place in an airtight container, cover, and chill the cookie dough balls for at least 2 hours, up to 1 day in advance.
Preheat The Oven & Prep Your Pans. When you’re ready to bake, remove the dough from the refrigerator while the oven fully preheats to 350 degrees F. Line two baking sheets with parchment (so you can alternate sheets between batches.)
Bake & Cool The Cookies. Roll 6 cookie dough balls in between your palms to smooth them out and place onto a prepared baking sheet. Bake at 350 degrees for 10-12 minutes, or until the edges are a golden brown color and the centers look just underdone. Garnish right away with a few extra chocolate chips, toffee bits, and a little flaky sea salt, if desired. Cool on the baking sheet for 2 minutes before transferring to a wire rack to finish cooling. Repeat process with remaining cookie dough.
Serve & Store. Enjoy right away once the cookies are cooled down, or store in an airtight container 2-3 days. (Or, freeze up to 2 months)