Be sure to take the time to sift your flour; it makes the cake light and airy.
Preheat your oven to 350°F. Line a muffin tin with baking cups or tulip cupcake liners.
Into a large bowl, measure out 1 1/2 cups of flour, making sure to remove 2 tablespoons. Add the cornstarch (or arrowroot starch) along with the baking powder and salt. Give it a quick whisk to combine.
Sift no less than three times. For best results, sift a total of five times. It is helpful to sift the flour mixture onto a large sheet of parchment paper. That allows you to pick up the paper and pour the flour into the bowl. Set the sifted flour aside.
In the bowl of an electric stand mixer, add the room temperature butter and sugar. Cream together until light and fluffy, for about 1 to 2 minutes. Scrape down the sides of the bowl. Add the eggs, one at a time and beat until well incorporated. Add the vanilla. Working in batches, add the buttermilk and dry ingredients alternately. Mix until incorporated.
Fill each baking cup about 2/3 full. Top with blueberries and about 3 apple slices, tucking them gently into the batter. Sprinkle with sanding sugar.
Bake in the center of the oven for about 25 to 30 minutes or until a toothpick inserted in the center comes out clean. Transfer to a wire rack to cool.To turn this recipe into a cake, lightly grease a 9-inch spring form pan. Bake for about 45-55 minutes or until a toothpick comes out clean when inserted. Cool on a wire rack before serving.
Instead of cupcakes, this recipe can be baked as a cake. Pour the prepared batter into a lightly greased 9-inch spring form pan. Arrange the blueberries and apple slices evenly over the surface, followed by an even sprinkling of the sanding sugar.
Bake for about 45 to 55 minutes or until a toothpick comes out clean when inserted in the center. Cool on a wire rack before serving.