In stand mixer, add butter and sugar. Blend with paddle attachment or electric beaters until mixture is light yellow. Add vanilla. Sift in flour and salt. Add small pieces of apple and continue blending, using hands, until incorporated. Remove dough, use a little flour (if needed), and work into a long roll (dough log). Wrap dough in plastic and chill for 45–60 minutes until butter sets. Preheat oven to 350 degrees. Remove dough from fridge. Slice into 1/2-inch circles and place onto ungreased baking sheet. Bake for 20 minutes or until golden. Remove from oven and cool. Cookies will harden as they cool. Top with apple cider and mascarpone frosting.