Make the cake: Preheat the oven to 350 degrees F. Line a 9-by-13-inch baking dish with foil, leaving an overhang on 2 sides; coat with cooking spray.
Whisk the flour, pie spice, baking powder, baking soda and salt in a medium bowl. Whisk the melted butter, both sugars, sour cream, eggs and vanilla in a large bowl until smooth. Stir the grated apple and the flour mixture into the butter mixture with a rubber spatula until combined.
Spread the batter in the prepared pan and bake until the top springs back when gently pressed, 25 to 30 minutes. Transfer the pan to a rack and let cool completely.
Make the frosting: Combine the heavy cream, sour cream, confectioners' sugar and vanilla in a large bowl and beat with a mixer on medium speed until thickened, about 1 minute. Increase the speed to medium high and beat until stiff peaks form, about 3 more minutes.
Transfer the frosting to a large piping bag fitted with a flat tip. Lift the cake out of the pan using the foil overhang. Cut into rectangles and transfer to a platter. Pipe the frosting on top in a zigzag pattern to look like bandages. Add candy eyeballs.