12 T. unsalted butter, plus more for pan

1 1/2 c. all-purpose flour

1 tsp. baking powder

1 c. light-brown sugar, firmly packed

2/3 c. granulated sugar

2 lg. eggs

1 tsp. pure vanilla extract

1 c. (6-oz.) butterscotch morsels

Preheat oven to 350°. Butter a 9-inch-square cake pan, and set aside. In a medium bowl, whisk together flour, and baking powder; set aside. In the food processor bowl, combine butter and brown and granulated sugars. Beat until light and fluffy, 2 to 3 minutes, scraping sides occasionally. Add eggs and vanilla; beat until well combined. Add flour mixture; beat just until combined. Fold in butterscotch chips. Using an offset spatula, spread batter into pan. Bake in preheated oven for 45 to 55 minutes, until surface is golden brown and a cake tester inserted into center comes out barely clean.