In a large bowl, combine the melted butter, granulated sugar, and light brown sugar. Mix until the mixture is smooth and well combined. Then, add the egg and vanilla extract, continuing to mix until they are fully incorporated into the sugar mixture.
Next, add the all-purpose flour, salt, and baking soda to the wet ingredients. Mix on low speed or by hand until the dry ingredients are just combined; be careful not to over-mix. Finally, use a spatula to gently fold in the chocolate chips and Halloween sprinkles until they are evenly distributed throughout the dough.
Using an ice cream scoop or two spoons, portion the dough into 2-tablespoon-sized balls. Place the dough balls onto a baking sheet lined with parchment paper. At this stage, they can be close together, as you are only chilling them.
Cover the baking sheet with plastic wrap and place it in the refrigerator to chill for at least 1.5 to 2 hours, or up to 24 hours. Chilling the dough is a crucial step that prevents the cookies from spreading too much while baking.
Once the dough has chilled, preheat your oven to 350°F (175°C). Arrange the cold cookie dough balls on a parchment-lined baking sheet, ensuring they are about 2 inches apart to allow for spreading. Bake for 11-13 minutes, or until the edges are golden brown.