In a mixing bowl of a stand mixer with the paddle attachment, cream the sugar and butter until light creamy and pale in color. You can use an electric mixer. This should take about 3 to 5 minutes for the best results. Add the egg and keep mixing at high speed until fully incorporated, add the peppermint extract and mix well.
In another mixing bowl, whisk the flour, cocoa powder, leavening agent, and salt to form a uniform color.
Sift the flour mixture into the bowl containing wet ingredients. Mix with a spatula then use your hands to make a soft dough.
Place the formed dough into a greased bowl and cover with a plastic wrap. Chill the dough for 30 minutes.
On a clean floured surface, roll out the dough into a rectangle of ½ inch thickness. Use a round cookie cutter to cut out the circle. Use a 3inch cookie cutter so they can have enough space to fit an After Eight Chocolate. Preheat the oven to 150C. Carefully place an After Eight Chocolate in the middle of each cutout cookie. Wrap it with the cookie dough, covering the After Eight Chocolate.
Arrange the Cookies on a baking sheet lined up with baking paper, and leave 2 inches apart, that's to allow spreading of the cookies as they bake.
Bake them for 12 to 15 minutes in the middle rack position in the oven. Once ready, let them cool on the baking tray for 5 minutes before carefully transferring them onto a cooling rack.
Place an After Eight chocolate on top of each baked cookie. Enjoy the cookies while warm with a cold glass of milk or for dessert after dinner.