While the cupcakes are baking, prepare the apple topping. Heat the butter in a medium-large skillet. Stir in the apples, sugar and cinnamon. Heat until the fruit begins to release its juices. Add ¾ cup of water to the pan and bring the mixture to a low simmer. Cook until the apples are just fork tender but not mushy. In a small bowl, combine the remaining 2 tablespoons of water with the cornstarch, and whisk lightly until smooth. Add the cornstarch mixture to the apples and cook 1-2 minutes more, until the mixture has thickened. Let cool thoroughly before topping the cupcakes.
In the bowl of an electric mixer fitted with the paddle attachment, beat the butter on medium speed for about 5 minutes. Turn the mixer down to low speed and gradually add in the confectioners’ sugar until incorporated. Add in the vanilla and salt and mix until incorporated. Turn the mixer back up to medium-high speed and beat for an additional 4 minutes, or until light and fluffy.
*Nutritional information is an estimate. Please consult a doctor or nutritionist for the most accurate nutritional info.