Preheat the oven to 350 degrees F. Line two 12-cup muffin tins with paper liners.
Prepare the cupcakes according to the cake-mix package directions, adding the cinnamon and nutmeg to the bowl with the dry mix.
Fill the muffin cups 3/4 full with the prepared batter and bake for 17 to 20 minutes, until a wooden skewer inserted into the center comes out mostly clean. A few moist crumbs are fine.
Remove the cupcakes from the tins and allow to cool completely on a wire rack.
Apple Pie Filling
In a medium-sized saucepan, melt the butter. Add the remaining ingredients, stirring until combined and the apples are coated with the mixture.
Cook over medium heat until the apples soften, about 12 to 15 minutes. Stir frequently and lower heat when the filling begins to bubble. Cook until the sauce thickens and begins to evaporate.
Remove from heat and cool completely.
Buttercream Frosting
Using a stand mixer, beat the butter until creamy, about 3 minutes on medium-high speed. Reduce the speed to low and add the cinnamon and salt. Once combined, slowly add the confectioners’ sugar.
Once the sugar has been incorporated, add 3 tablespoons of the defrosted apple juice concentrate. Increase the speed to medium high and beat until light and fluffy. If needed, add additional apple juice, 1 teaspoon at a time, until the desired consistency is reached.
Transfer the buttercream to a piping bag fitted with a large star tip.
Putting it together:
Using a rounded teaspoon measuring spoon, scoop out the centers of the cupcakes.
Spoon the cooled filling into the centers, packing it in.
Using a large star tip, pipe swirls of the buttercream over the tops of the cupcakes. Sprinkle with demerara sugar and garnish with cinnamon sticks if desired.