Preheat oven to 350℉. Spray 9 inch baking pan with non-stick spray and set aside.
For Apple and Walnut Layer
In a frying pan sprayed with non-stick spray ¼ cup butter, add maple syrup and heat over medium low heat until golden and bubbling, being careful not to burn. Add sliced apples and walnut halves and stir until well coated. Transfer mixture to prepared baking pan.
For Cake Layer
Whisk together flour, baking powder and salt, set aside.
In bowl of electric mixer beat ¾ cup sugar and ⅓ cup butter until light and fluffy. Add eggs and vanilla, beat until well combined.
Add flour mixture and milk alternately, mixing well after each addition. Continue beating for one minute. Pour batter over apples and walnuts, carefully spread to completely cover. Bake at 350℉ for 30-35 minutes until cake is golden and springs back when pressed.
Remove from oven and allow to cool for a full 10 minutes. Place serving plate upside down over the baking pan and carefully turn upside down to release cake to the plate. Serve warm or allow to cool before serving.
Notes
Store leftover cake in an airtight container in the refrigerator for up to 2 days.