In a medium sized saucepan over medium heat, melt the butter.
After it's done melting, leave on the heat and gently stir. You'll start to see little specks of brown forming. Once it's amber in color and has a nutty smell, remove from the heat.
While the butter is cooling, in a large bowl whisk together the flour, baking soda & salt. Set aside.
Using an electric mixer on medium speed beat together the butter, brown sugar and granulated sugar in a large bowl.
Add in the egg, egg yolk and vanilla and continue beating.
Turn the mixer down to low and slowly add in the flour mixture while beating, followed by the milk.
Turn off the mixer and stir in the butterscotch chips.
Form the dough into balls about 1 to 1.5 tablespoons in size. I got 30 cookies total. Place on a plate, cover with clingfilm and refrigerate for at least 3 hours or up to 48 hours.
Preheat the oven to 350F degrees. Place the cookies 2 inches apart on a cookie tray lined with parchment paper or a silicone baking mat, and flatten the cookies slightly.
Bake for about 8-10 minutes, or until the tops look almost set. Remove from the oven and optionally dot the tops of each cookie with a few chocolate chips.
Allow to cool on the tray for 10 minutes, then transfer to a wire rack to continue cooling.