Preheat oven to 350°F and line a baking tray with aluminum foil
In a small bowl, add all purpose flour, cocoa powder, salt, baking soda, and milk powder. Whisk together until evenly combined
In a medium bowl, use a hand mixer or stand mixer fitted with paddle attachment to cream together butter, granulated sugar, and brown sugar until well-incorporated. Add the egg, vanilla, and milk. Mix until well-incorporated, scraping down the sides of the bowl as needed.
Add 1/2 of the dry ingredients to the wet, and mix on low speed until incorporated. Then, add the remainder of the dry ingredients and mix on low speed until a cohesive dough forms. It should be quite thick.
Fold in the chopped semi-sweet chocolate. Save a few pieces to press onto the top of the cookies
Use a cookie or ice cream scoop to form balls of cookie dough about 2 inches wide (70g in weight). You should be able to make about 15 cookie dough balls. Press a white rabbit candy into the top of each ball, along with 1 - 2 pieces of chopped chocolate.
Place the cookie dough balls 2 inches apart on the baking sheet, as they will spread in the oven. Place in the middle rack and bake for 9 minutes.
After 9 minutes, open the oven door and lift the edge of the baking tray a few inches up and let it drop. Repeat a few times. The edges of the cookies will crinkle up and create a crispy edge.
Every 2 minutes, check the cookies and bang the pan. You should do this in total 2 - 3 times, until the cookies have been in the oven for a total of 14 - 15 minutes. They'll be done when the edges are crisp but the center is still soft and puffed up.
Remove from the oven and let cool on the baking tray for 5 minutes until they have firmed up. Place on a wire tray to finish cooling.