Preheat the oven to 350°F. Line two baking sheets with parchment paper.

In a microwave-safe bowl, combine the butter and ¾ cup semisweet chocolate chips. Microwave in 20-second intervals, stirring between each, until completely melted and smooth. Let cool for 5–10 minutes.

In a large mixing bowl, whisk together the brown sugar, granulated sugar, egg, egg yolk, and vanilla extract until smooth and slightly thickened, about 1 minute.

Pour in the slightly cooled melted chocolate mixture and whisk until fully combined.

Add the flour, cocoa powder, baking soda, and salt. Fold gently until a thick, soft chocolate dough forms. Avoid overmixing.

Fold in ½ cup of the holiday confetti mix and the additional chocolate chips, if using. Reserve the remaining ¼ cup of confetti for topping.

Refrigerate the dough for 15–20 minutes, just until firm enough to scoop. The dough will thicken as the melted chocolate cools.

Scoop approximately 2 tablespoons of dough per cookie onto the prepared baking sheets, spacing them about 2 inches apart.

Gently press the reserved holiday confetti onto the tops of the dough balls.

Bake for 10–12 minutes, until the edges are set and the centers look slightly soft. Do not overbake.

Let the cookies cool on the baking sheets for 10 minutes before transferring to a wire rack. The centers will continue to set as they cool.