2large firm-sweet apples (we like Honeycrisp and Jonathan), cored and cut into ½-inch cubes
Preheat the oven to 350 degrees F. Butter an 11-by-7-inch baking pan.
In a medium bowl, whisk together the flour, cinnamon, baking powder and baking soda. Set aside.
In the bowl of your mixer, beat together at high speed the cooled and melted butter, sugar and egg until pale yellow. By hand, stir in the walnuts and apples until evenly coated. Add the flour mixture, and stir until combined.
Spread the batter in the prepared pan and bake until golden brown and just firm to the touch, about 45 minutes. Let cool on a rack for 30 minutes. Cut into squares and store in a tightly sealed containers. Flavors are best one day after baking.