Preheat the oven to 350° and spray a 9-by-13-inch baking pan with baking spray. In a small bowl, whisk the flour with the baking powder, baking soda and salt. In a standing mixer fitted with the paddle, beat the butter with the brown and granulated sugars at medium speed until fluffy. Add the eggs and vanilla and beat until smooth. Add the dry ingredients and beat at low speed until incorporated. Beat in the pecan halves. Spread the batter in the prepared pan and bake for about 30 minutes, until shiny and lightly crackled on top and the edges pull away from the sides of the pan. Let cool completely.