Place butter in a saucepan over medium heat. Stirring constantly, and brown the butter. When the butter starts to foam and begins turn brown, turn the heat down to low, add milk powder, and stir for another 30-60 seconds. Then remove from heat to prevent burning. You should have about ⅔ cup butter (140 grams) after it is browned.

Let butter cool slightly before pouring butter into a bowl—make sure you get all the toasty brown bits at the bottom of the pan. Chill the butter in the fridge or freezer until it returns to room temperature—solid, but soft enough for a finger to indent into.

Preheat the oven to 350 degrees Fahrenheit. Place butter in a large mixing bowl or stand mixer with dark brown sugar and granulated sugar. Use a hand mixer or if using a stand mixer, use the paddle attachment, to mix on low for one minute until the butter and sugars are smooth.

Add in egg, egg yolk, and vanilla and mix on high for 2-3 minutes until the mixture pales in color and the texture is light and fluffy.

Add the dry ingredients to the creamed butter—cornstarch, flour, baking soda, salt, and cinnamon. Mix on low, scrape down the sides and the bottom of the bowl, until there are no streaks in the dough.

Add the oats and toffee to the dough and stir in until evenly dispersed throughout the dough.

Line a baking sheet with parchment paper or silicone baking mat. Scoop ¼ cup of dough and place on baking sheet, leaving plenty of room for the dough to spread. Sprinkle a little flaky sea salt over top of the dough.

Bake for 8-9 minutes, the cookies may look slightly underdone in the center—don’t worry, they will set up as they cool. Let cookies cool for 5 minutes on baking sheet before transferring them to a cooling rack.