Preheat oven to 350˚F (175˚C). Line a baking sheet with parchment paper or a baking mat.
In a medium mixing bowl or a stand mixer, beat 115 g unsalted butter until light and fluffy (about 2-3 minutes). Add 1 box cake mix and 1 large egg and mix on low speed just until combined. Fold in 60 g white chocolate chunks until evenly distributed.
Scoop out ¼-cup portions of dough onto the baking sheet, leaving 2-3" (5-7.6 cm) between. Press a few extra white chocolate chunks or chips and sprinkles into the top of each cookie if you like.
Bake 12-15 minutes, or until spread and just starting to turn lightly golden brown around the edges. See note 2 if making smaller cookies.
Leave to cool on the sheet for 5-10 minutes, then transfer to a wire rack to cool completely.
