For the cake: Place an oven rack in the center of the oven. Preheat the oven to 350 degrees F. Liberally butter and flour a 10-inch fluted tube or Bundt pan. Set aside.
In a large bowl, mix together the apples, maple syrup, eggs, sugar, oil, and vanilla. In a separate medium bowl, whisk together the flour, baking powder, baking soda, cinnamon, nutmeg, ginger, and salt. In batches, mix the dry ingredients into the apple mixture. Stir in the walnuts. Pour the mixture into the prepared pan and bake until a cake tester or skewer inserted into the cake comes out clean, 55 to 60 minutes. Cool for 15 minutes and invert onto a wire rack to cool completely, about 1 hour.
For the icing: In a food processor, combine the powdered sugar, heavy cream, cream cheese, 3 tablespoons water, and vanilla. Process until smooth, adding more water, 1 teaspoon at a time, until the icing is pourable. Spoon the icing over the cake and allow to set for 20 minutes before slicing.