In a large bowl, whisk together all purpose flour, cocoa powder, baking soda, and kosher salt.

In a separate mixing bowl or the bowl of a stand mixer, mix softened butter on medium-high speed until until creamy, about 30 seconds.

Add the granulated sugar and dark brown sugar, followed by the egg, milk, and vanilla extract. Mix until thoroughly combined, about 2 minutes more. Mixture might look slightly separated.

Add the dry ingredients and mix on low speed until a soft dough forms. Do not overmix.

With a flexible rubber spatula, fold in the chocolate chips, M&Ms, and chopped chocolate bars.

Cover bowl with clingfilm/plastic wrap. Refrigerate until firm enough to scoop, at least 2 hours, or overnight.

Line a baking tray with parchment or a silicone baking mat.

With a 1 1/2-inch cookie scoop or melon baller, scoop the dough into 1 1/2-inch portions. Arrange half of the portions on prepared tray, allowing 2 inches of space in between each ball for the cookies to spread as they bake.

Freeze tray for 15 minutes. Meanwhile, preheat oven to 350° F / 175° C.

Bake in preheated oven for 14-15 minutes, or until cookies are puffy and set around the edges but still somewhat soft in the middle.

For perfectly round cookies, quickly reshape each cookie with a round metal cutter.

Working quickly, press a few candy corn and chopped candy bar pieces into the tops of each cookie to decorate.

Allow cookies to cool on the tray for 10 minutes, then transfer to a wire rack to cool completely.

Repeat with remaining dough.

Store cookies in an airtight container at room temperature for up to 3 days.