Preheat the oven to 350°F/180°C. Line the bottom of a 12x17-in. baking pan with parchment paper.
With a hand or stand mixer, cream butter and sugar together on medium speed until fluffy, about 3 minutes.
Reduce speed to low. Add eggs to butter mixture one at a time, incorporating each egg fully. Add vanilla and mix; set aside.
Sift flour, baking powder and salt in a bowl. Add gradually to egg mixture, mixing on low speed, scraping bowl as needed until completely incorporated.
Turn off mixer and fold in butterscotch chips. Spread batter evenly in the baking pan. Sprinkle top with pecans.
Bake for 25 - 30 minutes. Remove from oven and cool slightly before cutting into squares.