
For the cookies: Preheat the oven to 350˚. Lightly butter a 9-inch square baking pan. Line the pan with parchment paper, leaving an overhang on all sides. Whisk the flour and salt in a medium bowl; set aside.
Beat the butter and cream cheese in a large bowl with a mixer on medium speed until smooth and fluffy. Add the granulated sugar and beat until smooth. Add the egg and vanilla and mix until fully incorporated, scraping down the bowl as necessary. Reduce the mixer speed to low and slowly beat in the flour mixture until combined.
Transfer the dough to the prepared pan and gently press into an even layer with your fingers. Bake until the edges are lightly golden, about 15 minutes. Let cool completely in the pan, then lift the bars out of the pan using the parchment overhang.
Meanwhile, for the frosting: Combine the butter, powdered sugar and milk in a large bowl and beat with a mixer on medium speed until light and fluffy. Beat in the peppermint extract. Spread the frosting over the cooled bars and sprinkle with the candies. Cut into pieces.