Mix flour, cocoa, and baking powder in a bowl. Beat softened butter, confectioner’s sugar, vanilla, and a pinch of salt in a large mixing bowl until foamy. Then beat in an egg. Add dry ingredients and give it a quick beat. At last, add half the peppermint oil drop by drop. Roll the dough into a log, wrap in plastic, and chill for approx. 1 hr.
Pre-heat oven to 175°C/350°F. Cut the log into 1-cm/0.5 in. thick circles. Lay them onto a lined baking sheet. Bake cookies for approx. 10 – 12 min. Let cool for approx. 30 min.
If using chocolate bars, chop the chocolate into pieces. Melt the dark chocolate in a bowl set over a pot with simmering water. Add the remaining peppermint oil drop by drop to taste, stirring thoroughly after each drop. Dip the cooled cookies into the chocolate completely, and let drip for a few seconds. Lay onto a piece of parchment paper or a cooling rack. Melt white chocolate. Use a fork to drizzle it over the cookies. Store in the fridge, and enjoy cold!