Heat oven to 350°F. Coat 9” round baking pan with nonstick spray. In small bowl, stir together butter and ½ cup grated piloncillo until well combined. Spread evenly into prepared pan.. Thinly slice one apple and arrange in pan in concentric circle; set pan aside.
Finely chop remaining apple half (you’ll have about ¾ cup) and transfer to medium bowl. Whisk in egg, oil and remaining grated piloncillo until well combined; set aside.
In separate medium bowl, using whisk, whisk together flour, cinnamon, nutmeg, baking soda, baking powder and salt. Pour half of flour mixture into bowl with apples, stirring until just combined. Add remaining flour mixture to bowl and stir until just combined. Pour apple batter into reserved pan. Using spatula, evenly spread batter in pan.
Bake until toothpick inserted into center of cake comes out clean, about 35 minutes. Transfer apple cake to cooling rack until cool enough to handle, about 5 minutes. Using thin knife, run knife around edges of pan. Invert cake onto serving plate. Serve warm with whipped cream and milk caramel.