Line a small baking tray or sheet pan with parchment paper, then set it aside.

In a large bowl, beat the softened butter together with the brown sugar and granulated sugar until the mixture becomes pale and fluffy.

Add the room-temperature egg and vanilla extract to the creamed mixture, then beat until fully incorporated.

In a separate bowl, whisk together the all-purpose flour, cornstarch, baking soda, and salt until evenly combined.

Gradually add the dry mixture to the wet ingredients, mixing only until a dough comes together. Avoid overworking the dough.

Fold in the semi-sweet chocolate chips and candy-coated chocolates, distributing them evenly throughout the dough.

Portion the dough into balls of about 2 tablespoons each using a cookie scoop or your hands, then place them on the prepared baking sheet. For a more decorative finish, press a few extra candies onto the tops.

Cover the tray with plastic wrap and freeze the dough balls for at least 1 hour or up to overnight. When chilled overnight, let the dough sit at room temperature for about 10 minutes before baking. The dough balls may also be transferred to a zip-top bag and baked as needed.

Preheat the oven to 350°F (175°C). Arrange the chilled dough balls on a parchment-lined baking sheet, leaving space between each one to allow for spreading.

Bake for 10 to 12 minutes, until the cookies are set around the edges but still appear slightly underdone in the center.

While the cookies are still warm, press a few extra chocolate chips or candy pieces into the tops if desired.

Leave the cookies on the baking sheet for several minutes to firm up, then transfer them to a wire rack to cool completely.