Preheat the oven to 350 F (177 C). Grease and line the bottom of a 9 inch (23 cm) cake pan with parchment paper.
In a mixing bowl whisk the melted butter with the light brown sugar until a paste forms. Evenly spread the paste across the bottom of the prepared pan.
6 tablespoons (85g) unsalted butter, melted, ½ cup (100 g) light brown sugar, ¼ teaspoon ground cinnamon , ¼ teaspoon vanilla extract
Wash, dry, peel, core and slice the apples into ¼ inch slices. Just try to keep the slices as similar as possible to make the layering easier. Decoratively layer the apple slices across the bottom of the pan, fully covering the sugar paste.
2 cups peeled apple slices
In a large mixing bowl, whisk together the flour, baking powder, apple pie spice and salt together until well combined.
1½ cups (180 g) all purpose flour, 1½ teaspoons baking powder, 1½ teaspoons apple pie spice , ½ teaspoon salt
In a large mixing bowl fitted with paddle attachment, cream together the butter and sugar and light brown sugar on medium speed for 3 -4 minutes. Scrape down the sides and bottom of the bowl. With the mixer on low, cream in the eggs and vanilla extract until well combined. Scrape the bottom and sides of the mixing bowl.
6 tablespoons (85g) unsalted butter, room temperature, ½ cup (100 g) granulated sugar, ¼ cup (50 g) light brown sugar, 2 large eggs, room temperature, 2 teaspoons (10 ml) vanilla extract
Finally, with the mixer on low, alternate mixing the dry ingredients and buttermilk into the wet ingredients in this order: dry, buttermilk, dry, buttermilk, dry. Evenly spread the buttery spice cake batter into the prepared pan on top of the apples. Make sure to fully cover the apples with the cake batter. ¾ cup (180 ml) buttermilk, room temperature
Bake the apple upside down cake for 40 - 50 minutes or until a toothpick inserted into the center of the cake comes out clean. Cool the cake pan in the pan for 10 - 15 minutes. Use a butter knife to gently separate the edges of the cake from the pan. Place a serving platter or large plate over the top of the cake pan. Invert the cake pan to the top is now on the bottom of the platter or plate. Gently lift the pan off of the cake to reveal the parchment lined apples. Gently remove the parchment paper from the apple layer. The cake is now ready to be served. I recommend serving this with freshly whipped cream or a scoop of ice cream and a drizzle of salted caramel.