Brown the butter and let it cool slightly. Please see the Recipe Tips and Tricks for more detailed instructions on how to brown butter.
1 cup (2 sticks or 227 grams) unsalted butter
While the butter is cooling, line two baking sheets with parchment paper or silicone mats.
Whisk or sift together the flour, salt, baking powder, and baking soda in a small bowl.
2 cups (250 grams) all-purpose flour, 1 teaspoon kosher salt, ½ teaspoon baking powder, ½ teaspoon baking soda
In a large mixing bowl or bowl of a stand mixer, beat the brown sugar and melted butter until well mixed, about three to five minutes.
1¼ cups (240 grams) dark brown sugar
Add the eggs one at a time and beat for one minute after each addition, scraping down the bowl as necessary. Add the milk and vanilla and beat for another minute.
2 eggs, 1 tablespoon milk, 2 teaspoons vanilla extract
Add the bowl of dry ingredients and mix on low just until all traces of flour disappear. Be careful not to over-mix.
Stir in the butterscotch chips and semi-sweet chocolate chips by hand. At this point you can chill the bowl of butterscotch cookie dough in the refrigerator for one hour, or scoop out 12 cookies on a lined cookie sheet and put it into the freezer for 15 to 30 minutes.
1 cup (170 grams) butterscotch chips, 1 cup (170 grams) semi-sweet chocolate chips
Heat your oven to 375℉ and bake for 11 to 14 minutes, until the cookies are set and the edges are just starting to brown. If you are baking two cookie sheets at the time, check your cookies after about eight or nine minutes and rotate them as necessary.
Cool the butterscotch cookies on the baking sheet for five minutes, then transfer them to a baking rack to cool completely.