Preheat oven to 350° Fahrenheit. Thoroughly grease a 9” cake pan with butter. Peel, core, and slice apples into 1/4” pieces, and place in a bowl. Heat maple syrup to medium low heat to reduce. Once boiling, set a timer for 10 minutes. Maple syrup may boil over, so do not leave the kitchen. Pour thickened maple syrup into prepared cake pan and swirl around to reach the entire bottom surface. Arrange the apples in the pan in two concentric circles, overlapping them slightly.
In a stand mixer fitted with a paddle attachment, cream butter and sugar together. Add in eggs, and vanilla. Scrape down sides as needed. Next, add flour, Clabber Girl® Baking Powder, and salt. Add milk last and scrape down sides as needed. Once batter is made, pour over the apples and use a spatula to smooth it out in an even layer. Bake for 50 minutes or until the cake is golden brown and a toothpick inserted comes out clean. Let the cake cool at room temperature for 5 minutes, then invert onto a serving dish.