Line a baking sheet with plastic wrap or clingfilm allowing a little overhang.
To make the butter mixture, in a large bowl, use a baking spatula or wooden spoon to cream the butter and sugar. Add the egg, vinegar, baking soda and salt. Mix well.
Transfer half of the butter mixture into a large bowl and add the Peppermint Spice Drops® and green food colouring to it. Combine well and set aside. This will be
for the mint dough.
To make the chocolate dough, use the rest of the butter mixture and add the Vanilla Spice Drops® and cocoa powder. Mix well. Gradually stir in 375g cups flour. When the dough gets too stiff, use your hands to knead it into a smooth ball. If the dough is too sticky, knead in a teaspoon of flour.
Press down the chocolate dough to cover the bottom of the clingfilm covered baking sheet in a thin layer. Dust hands with flour, if needed to make it thin and smooth. Chill for about 10 minutes in the refrigerator.
While the chocolate dough chills, finish making the mint dough. Take the mixture for the dough and gradually stir in the 280g flour. Knead into a smooth ball.
Roll the mint dough and place it on the chilled chocolate dough. Use the plastic to lift the pressed dough out of the pan and onto your work table. Keep the plastic under the dough. To make it easy to roll, use the heels of your hands to flatten one side of the dough to 1/4 inch thick. Place this flattened strip in front of you to begin rolling. Lift the plastic to help you roll the dough into a tight cylinder.
Pat the ends in. Wrap the dough in the plastic wrap. If the cylinder is thicker in some parts than others, roll the thicker parts a few times to even it out. You should have a cylinder that is 2 1/2 inches in diameter. Chill for 30 minutes.
Preheat the oven to 175°C/345°F/gas 3.
After chilling, remove the plastic, and then use a knife to cut the cylinder into 1/2 inch thick rounds. If the dough is too stiff to cut, let it warm up a little first. Place the rounds on a lined baking sheet at least 3 inches apart.
Bake the cookies for about 16 minutes, or until the edges are firm, the tops are no longer shiny and the green dough is still green (if baked for too long, the green dough will turn golden). They might be slightly soft in the middle but will firm up once cooled. Cool completely on the sheet and then serve. Store in an airtight container.