Preheat oven to 400 degrees.
Line a cupcake pan with baking liners.
In a large mixing bowl, whisk together the dry ingredients. In a small bowl, whisk together the oil, vanilla extract, and the eggs. Pour the egg mixture into the dry ingredients and stir until everything comes together. Fold in the diced apples. Divide the batter evenly between the muffin liners. Bake for 22-25 minutes or until the tops are golden and a toothpick inserted into the center comes out clean. Remove the cupcakes from the oven and let cool completely.
To make the caramel sauce (can be made ahead of time):In a small saucepan, whisk together the sugar and water. Bring to a boil over medium heat, stirring continuously. Once the mixture is boiling, stop stirring. Continue to boil until the sugar turns an amber color. Remove the pan from the heat and add in the butter and salt (this will bubble, so be careful). Whisk until all the butter has been incorporated, then add in the heavy cream. Again, the mixture will bubble, so whisk this carefully. If the caramel turns hard, put the pan over low heat and stir until the caramel has melted and the mixture is smooth. Strain the caramel sauce through a sieve into a heat-safe container. Refrigerate until needed.
To make the caramel buttercream:In a stand mixer using the whisk attachment, cream together the butter and cooled caramel sauce. Add in the powdered sugar, 1 cup at a time. Slowly add in the heavy cream until you have reached the desired frosting consistency.
Spoon the caramel buttercream into a pastry bag fitted with a star tip. Frost the cooled cupcakes, then drizzle on additional caramel sauce. Enjoy!