To prepare a white chocolate and walnut pumpkin bars recipe, first you'll need to prepare your ingredients. First, preheat your oven to 350?F (180?C) and grease and flour two 9x9-inch baking pans. Then, mix together the flour, sugar, baking powder, salt, and pumpkin pie spice. Add the dry ingredients to the wet ingredients and stir to combine. Pour the batter into the prepared pan and bake for 20 to 25 minutes, or until the top is browned and the center is set. Allow the bars to cool in the pan for 10 to 15 minutes before cutting into squares and serving.