How to Make It
Preheat oven to 350°F. Grease a 13- x 9-inch baking pan with butter. Line pan with parchment paper, allowing paper to extend over edges of pan. Grease paper.
Prepare the Blondies: Combine butter and granulated sugar in a large bowl. Beat with an electric mixer on medium speed until well combined, about 3 minutes. Add eggs, 1 at a time, beating until combined after each addition. Beat in vanilla and salt. Add flour; beat on low speed just until combined, 1 to 2 minutes. Stir in white chocolate chips and 1/2 cup of the crushed peppermints, reserving remaining crushed peppermints.
Spread batter in prepared pan; smooth top with a spatula. Bake in preheated oven until a wooden pick inserted in center comes out clean, 20 to 22 minutes. Cool completely in pan. Lift blondies from pan using parchment paper as handles. Remove and discard paper; trim browned edges.
Prepare the Icing: Combine powdered sugar, butter, vanilla, and peppermint extract in a medium bowl; beat on medium speed until smooth. Beat in 2 tablespoons of the cream. Add up to 1 additional tablespoon cream, if necessary, until icing reaches a spreadable consistency. Spread Icing over top of blondies. Sprinkle with reserved crushed peppermints. Cut into 3-inch squares; cut squares into triangles.