Preheat the oven to 350 degrees Fahrenheit. Grease a large baking sheet, and set aside.
In a bowl of a stand mixer or using a hand mixer, cream together the melted browned butter, brown sugar, and granulated sugar.
1/2 cup melted browned butter, 1/3 cup dark brown sugar, 1/4 cup granulated sugar
Add the egg, egg yolk, and vanilla extract. Beat for 1 minute on medium speed.
1 large egg, 1 egg yolk, 1 Tablespoon vanilla extract
Add the rolled oats, flour, cinnamon, baking soda, and salt to the bowl of wet ingredients, and beat just until combined. Don’t overmix!
1 cup rolled oats, 1 cup all-purpose flour, ⅛ teaspoon cinnamon, 1/2 teaspoon baking soda, 1/2 teaspoon kosher salt
Fold the chocolate chips, peanut butter chips, and toffee bits into the mixture until evenly distributed.
1/2 cup semi-sweet chocolate chips, 1/2 cup peanut butter chips, 1/4 cup toffee bits
Scoop the dough into 1/2 cup portions, rolling them into balls. Place the dough on the prepared baking sheet, leaving space between each cookie. Use a palm to gently flatten each mound of dough.
Bake for 10-12 minutes or until the dough is lightly golden and the edges are set. Immediately sprinkle with sea salt.
Sea salt for topping
Transfer to a cooling rack.
Serve warm or at room temperature.