Step 1: Bake the Cinnamon Cupcakes

1. Preheat the oven to 350°F and line a cupcake pan with 12 liners.
2. In a bowl, whisk together flour, baking powder, cinnamon, and salt.
3. In a separate bowl, use an electric mixer to cream sugar, butter, and oil until smooth. Add the egg, sour cream, and vanilla extract, and mix until combined.
4. Gradually add half of the dry mixture, followed by the milk, and then the remaining dry mixture. Mix until smooth.
5. Fill the liners about ¾ full with batter and bake for 20 minutes, or until the tops bounce back when lightly touched. Cool completely on a wire rack.

Step 2: Prepare the Cinnamon Buttercream Frosting

1. Whip butter on medium-high speed for 5 minutes until pale and fluffy.
2. Gradually add the powdered sugar and mix on low speed until combined.
3. Add vanilla, cinnamon, and salt, mixing until smooth. Slowly drizzle in heavy cream and beat for another 3 minutes for a creamy texture.

Step 3: Make the Apple Cinnamon Filling

1. In a saucepan, combine apples, sugar, water, lemon juice, and cinnamon. Cook over medium heat for 10-15 minutes until apples soften.
2. Mix cornstarch and 1 tbsp water into a slurry. Remove the saucepan from heat, stir in the slurry, and mix well to thicken.
3. Let the filling cool to room temperature before using.

Step 4: Create the Crumble Topping

1. Combine flour, brown sugar, and cinnamon in a bowl. Use a fork or your fingers to incorporate the butter until the mixture resembles coarse crumbs.
2. Spread the crumble onto a parchment-lined baking sheet and bake at 350°F for 10-15 minutes, stirring every few minutes until golden brown. Cool completely.

Step 5: Assemble the Cupcakes

1. Pipe a ring of cinnamon buttercream around the edge of each cupcake using a piping bag with an Ateco 826 tip, leaving a well in the center.
2. Spoon the apple cinnamon filling into the well of each cupcake.
3. Sprinkle the tops with the crumble topping. For extra flair, drizzle a bit of caramel sauce over the top if desired.