01 - Preheat your oven to 350°F (175°C). Line a large baking sheet with parchment paper.
02 - In a medium bowl, whisk together flour, baking soda, and salt. Set aside.
03 - In a large bowl, whisk together melted butter, brown sugar, and granulated sugar until smooth. Beat in the egg and vanilla extract.
04 - Add the dry ingredients to the wet ingredients and stir until a soft dough forms.
05 - Gently fold in chocolate chips, crushed peppermint, pretzels, and nuts if using.
06 - Transfer the dough to the prepared baking sheet and press into a large, thin rectangle, approximately 1/4-inch thick.
07 - Bake for 18–20 minutes, or until golden brown and set at the edges. Remove from the oven and allow to cool slightly.
08 - While the brittle is still warm, use a pizza cutter or sharp knife to score it into irregular pieces. Let cool completely on the baking sheet.
09 - For the marshmallow drizzle, combine mini marshmallows and milk in a microwave-safe bowl. Heat in 20-second intervals, stirring between each, until smooth.
10 - Drizzle the melted marshmallow over the cooled cookie brittle. Allow to set for 10 minutes before serving or packaging.