Brown the butter: Melt the butter in a medium sized pan over medium heat. Once it melts, it will crackle, then become foamy – continue mixing the butter while this happens to prevent burning. The butter will go from melted to golden brown quickly, and you'll know it's almost done when the aroma of browned butter arrives. The whole process should take about 5 minutes! Once golden, remove from heat and transfer to a glass bowl to cool as you measure out the rest of your ingredients. (Cool at least 15 minutes).
In a large mixing bowl, whisk together the cooled brown butter and brown sugar. Then whisk in egg, egg yolk, and vanilla.
3/4 cups unsalted butter, 1 cup dark brown sugar, 1 egg, 1 egg yolk, 1 tbsp vanilla
Toss in the flour, oats, baking soda, and salt. Using a spatula, gently fold until no dry streaks show.
1 1/3 cups all-purpose flour, 1 cup large flake oats, 1/2 tsp baking soda, 1/2 tsp salt
Mix in the toffee pieces and chocolate chips.
1/2 cup semi-sweet chocolate chips, 1/3 cup toffee pieces
When you are ready to bake the cookies, preheat the oven to 350°F and place parchment paper on two baking trays.
Using a cookie scooper, scoop 6 cookies on the baking tray.
Bake for 11-13 minutes or until the edges are a light golden brown. Remove from oven and sprinkle with flakey sea salt (optional, but recommended).
Enjoy!