Preheat your oven to 350°F (175°C) and line a baking sheet with parchment paper.

In a mixing bowl, whisk together the flour, baking soda, and salt. Set aside.

In another bowl, cream the softened butter, brown sugar, and granulated sugar using an electric mixer until light and fluffy.

Add the eggs one at a time, mixing well after each addition, then stir in the vanilla extract.

Gradually add the dry ingredients to the wet ingredients, mixing until just combined.

Fold in the birthday cake mix and sprinkles until evenly distributed.

Scoop tablespoon-sized balls of dough onto the prepared baking sheet, spacing them about 2 inches apart.

Top each cookie with additional sprinkles if desired.

Bake in the preheated oven for 10-12 minutes, or until the edges are lightly golden but the centers are still soft.

Remove from the oven and let the cookies cool on the baking sheet for 5 minutes before transferring them to a wire rack to cool completely.