Preheat oven to 350° F.
Lightly spray an 8-inch pie plate with vegetable oil.
Beat egg, sugar, vanilla extract and baking powder in a large mixing bowl until smooth and fluffy.
Beat in flour until smooth and well blended.
Stir in walnuts and apples.
Turn into prepared pie plate and bake 30 minutes.
Pie will puff up as it cooks, then collapse as it cools.