1 1/2cupswhole wheat pastry flour OR all-purpose flour
1 1/2teaspoonsbaking powder
6tablespoonsunsalted butterat room temperature (plus additional for pan)
2teaspoonspure vanilla extract
1tablespoonssparkling sugarfor topping
Peel and slice each apple into 8 equal slices. Place in a plastic baggie, add a splash of lemon juice, seal the bag, and shake. Sprinkle a good amount of cinnamon sugar into the bag then seal and shake again until all of the apples are coated.
Position rack in center of oven and preheat oven to 350°F. Generously butter a 9" deep-dish pie plate.
In a medium bowl, whisk together flour, baking powder, cinnamon, and salt; set aside. In a large bowl, use an electric mixer to blend the butter until creamy. Add the sugar and beat on medium-high speed for a couple of minutes until light and fluffy. Turn down the mixer and slowly blend in the egg, buttermilk, and vanilla. With the mixer on low, gradually mix in the flour mixture until just combined.
Scrape the batter into the prepared pie plate and smooth the top. Arrange the apple slices on top of the batter, lightly pressing them down. Sprinkle the top of the cake with the sparkling sugar.
Bake for 10 minutes at 350°F. Then reduce the oven temperature to 325°F and continue baking for 50 to 60 minutes, or until cake tests done with a toothpick inserted into the center of the cake.
Place the pie plate on a rack and allow to cool. Slice into wedges and serve with fresh whipped cream or vanilla ice cream, if desired.