In a large bowl, whisk together the flour, cake mix, salt, and baking soda. Set aside.
In another bowl, beat the butter, brown sugar, and white sugar with an electric mixer until light and fluffy.
Add the egg, vanilla, and cake batter extract, then mix until everything’s well combined.
Gradually add the dry ingredients to the wet ingredients, mixing just until combined.
Fold in the white chocolate chips and Christmas sprinkles.
Cover the dough with plastic wrap and chill for at least 2 hours — don’t skip this step, it helps the cookies bake up thick and chewy.
When ready to bake, preheat your oven to 350°F and line cookie sheets with parchment paper.
Scoop about 2 ½ tablespoons of dough per cookie, shaping into balls that are slightly taller than they are wide.
Bake for 13–15 minutes, until the edges are lightly golden but the centers are still soft — that’s how you get that perfect chewy texture.
Let cookies cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely.
While the cookies cool, make the frosting: beat the butter with an electric mixer until fluffy, then mix in the extract.
Gradually add the powdered sugar, whipping until the frosting is light and fluffy.
Pipe onto the cooled cookies, top with extra Christmas sprinkles, and serve!