For the flour mixture - Combine the flour, cocoa powder, baking soda, baking powder and salt in a bowl and whisk well to combine.
Chop the Peppermint crisp bars into little chunks, or before opening the packets, crush the candy bar using a rolling pin.
Beat the butter and sugar on high speed using a stand mixer with a paddle attachment until it's pale and fluffy. Scrape the bowl and beat again for a few seconds. Add in the egg, vanilla extract and peppermint extract and beat for 30 seconds.
Reduce the speed to medium-low and add the flour mixture slowly. Once the dough comes together, add the chocolate chips and 1/2 CUP of the chopped peppermint crisp. You may need to increase the speed to high for a few seconds so the chocolate mixes into the batter. If you have time, wrap the cookie dough in plastic wrap and chill it for 15 minutes.
Preheat oven to 180°C (360°F). Line two baking trays with non-stick baking (parchment) paper. Scoop 2 tablespoons of cookie dough into your hands, then roll them into a ball. Dip the ball into the remaining chopped peppermint crisp, then place it on the baking tray. Space them approximately 1 inch apart. If you prefer flatter cookies, press the dough balls down gently with the palm of your hands.
Bake the cookies for 7 to 8 minutes. Sprinkle any remaining chopped Peppermint Crisp on top of the cookies, then allow them to cool completely on the tray, so they set.
Notes
(NOTE 1) Peppermint Crisp bars - I used 4 x 35gm (4 x 1 oz) Nestle's Peppermint Crisp bars. These bars have a crisp peppermint cracknel centre that's covered in milk chocolate. If you can't get these where you are, you could swap them for equal amounts of candy canes or add more chocolate chips.(NOTE 2) Plain flour (All-purpose flour) has no leavening (raising agents) such as baking powder mixed in.(NOTE 3) Caster sugar (superfine sugar) - Caster sugar has finer granules than regular white sugar, but it's not the same as icing/powdered sugar. Many baking or candy recipes call for caster sugar as it dissolves faster and better into mixtures. If you can't find caster sugar, you can make your own by placing regular sugar in a food processor or blender. Pulse the sugar a few times until it looks finer, but don't over-blitz it to a powder consistency. However, you could just use equal amounts of regular white sugar.Leftovers - Allow the cookies to cool completely at room temperature, then transfer them into an airtight container. Store them at room temperature for up to 3 days.I've used a third-party application to calculate the calories and nutritional information, so please use this as an approximate guide only.Cooking measurements are in Australian standard spoon and cup measurements. For specific details and conversions, visit ourAustralian Cooking Measurements page.