1. Preheat oven to 375ºF. Grease a 9 x 13 inch baking pan.
2. In a large bowl whisk together the flour, baking powder, baking soda, salt, 3 teaspoons Apple Pie Spice. Set aside.
3. In a second bowl lightly beat the eggs. Add in the oil, brown sugar, apple sauce and vanilla extract. Whisk well to combine, then add to the dry ingredients.
4. Fold in the grated apple, mixing well to combine. Spread batter into prepared pan. Give the pan one sharp tap (on the counter lined with a dish towel to prevent breaking) to remove any air bubbles.
5. Bake for 45-50 minutes or until a tooth pick comes out clean. Cool completely on a wire rack.
6. Frosting - In the bowl of a stand mixer fitted with a whisk attachment or using a handheld mixer, beat the cream cheese and butter until smooth and creamy, about 4 minutes.
7. Add in the confectioners' sugar, vanilla, salt and remaining 1 teaspoon Apple Pie Spice. Beat on low until all ingredients are combined, then switch to high for 3 minutes.
8. Spread frosting on the cake.