Because this is a cross between a candy and a cookie, cut it into small squares to serve.
Courtesy of Jennifer Low.
Preheat oven to 300 F (150 C). Line baking sheet with parchment paper.
To make chocolate shortbread base, mix flour and cocoa powder. Set aside.
In large bowl, using either electric handheld or stand mixer, beat butter until fluffy. Gradually beat in brown and white sugars and vanilla until blended. Gradually beat in flour mixture, switching to wooden spoon when dough is too stiff for mixer.
Pat dough into a disk. Place on parchment-line sheet. Roll dough as close to a square or rectangle shape as you can, 1/4-inch (5-mm) thick. Prick dough every 2 inches with fork. Bake on middle rack of oven 25 to 30 minutes, until top is no longer shiny. Remove from oven to cool completely.
Make top layer by melting white chocolate and butter in bowl set over simmering water. Mix in water. Remove from heat. Beat in icing sugar until smooth. Spread over cooled chocolate shortbread.
Sprinkle with crushed peppermint candy. Allow top layer to firm up at least 30 minutes at room temperature. Slide shortbread off parchment onto cutting board. Using a large sharp knife, trim edges to straighten sides (scraps can be eaten as cook’s treat). Cut trimmed piece into 1-inch squares.
Store in single layer in sealed container in cool dry place up to 2 days.