01 - Preheat oven to 350°F. Grease and line three 8-inch round cake pans with parchment paper.
02 - In a bowl, whisk together flour, baking powder, baking soda, salt, cinnamon, and nutmeg.
03 - In a large bowl, beat the softened butter, vegetable oil, granulated sugar, and brown sugar together until light and fluffy, about 3 minutes.
04 - Beat in eggs one at a time, mixing well after each addition. Stir in sour cream and vanilla extract until incorporated.
05 - Gradually add dry ingredients to the wet mixture, mixing just until combined. Fold in the diced apples.
06 - Divide batter evenly among the prepared pans. Bake for 28–32 minutes or until a toothpick inserted in the center comes out clean.
07 - Let the cakes cool in pans for 10 minutes, then transfer to wire racks to cool completely.
08 - In a medium saucepan over medium heat, melt the sugar while stirring constantly until it turns amber. Add butter and stir until melted. Slowly incorporate heavy cream and stir until smooth. Mix in sea salt and cool completely.
09 - Beat butter until creamy. Gradually add sifted powdered sugar. Blend in cooled salted caramel, then mix in heavy cream as needed for a smooth, spreadable consistency.
10 - Place one cake layer on a serving plate. Spread with caramel buttercream, sprinkle evenly with chopped Snickers, and drizzle salted caramel. Repeat with remaining layers.
11 - Frost the exterior of the cake with remaining buttercream. Chill assembled cake for 30 minutes for stability.
12 - Finish with additional salted caramel, extra Snickers pieces, and a light sprinkle of flaky sea salt before serving.