In a large mixing bowl, combine the butter, granulated sugar, and brown sugar. Mix until smooth and creamy.

Add the egg and vanilla extract to the bowl. Mix again until fully incorporated.

Sift in the self-rising flour, baking powder, salt, cinnamon, and cornstarch. Mix well, using your hands if needed, until the dough is smooth and consistent.

Gently fold in the chocolate chunks and mini M&Ms until evenly distributed throughout the dough.

Using a spoon or ice cream scoop, form dough balls and place them on a parchment-lined baking sheet. Ensure they are spaced about 5cm apart to allow room for spreading. If some cookies lack visible M&Ms, press a few on top gently for a festive look.

Cover the dough balls with plastic wrap and refrigerate for at least 30 minutes (or up to 48 hours for enhanced flavor).

Preheat the oven to 350ºF (175°C). Bake the cookies for 15–20 minutes, or until the edges are golden brown. The centers should look slightly undercooked. If they appear fully done in the oven, they are likely overbaked and will become dry once cooled.

Allow the cookies to cool on the baking sheet for 15 minutes before transferring them to a wire cooling rack to avoid breaking them.