In a large bowl, using an electric mixer on medium speed, beat butter, brown sugar, eggs and vanilla until thick and smooth, about 3 minutes. Combine flour, baking powder and salt. Add to butter mixture on low speed, mixing until blended. Stir in apples and nuts, mixing well. Spread evenly in the prepared tin and bake until set and golden, 25-30 minutes. Let cool completely in pan on rack.