Preheat the oven to 350 degrees F. Line a 9-by-13-inch baking dish with foil, leaving an overhang on 2 sides. Butter the foil.
Whisk the flour, baking powder and salt in a medium bowl; set aside.
Beat the butter, granulated sugar and brown sugar in a large bowl with a mixer on medium-high speed until fluffy, about 3 minutes. Add the vanilla and beat until combined. Add the eggs one at a time, beating after each addition. Reduce the mixer speed to low; beat in the flour mixture until just combined. Fold in 1/2 cup each butterscotch chips and pecans.
Transfer the batter to the prepared dish and press with your fingers into an even layer. Bake until the dough starts setting, about 15 minutes, then sprinkle with the remaining 1/4 cup each butterscotch chips and pecans. Continue baking until puffed and golden and a toothpick inserted into the center comes out clean, 20 to 25 more minutes. Transfer to a rack and let cool completely. Lift out of the pan, peel off the foil and cut into pieces.