Place the butter, cake mix, and eggs in a large mixing bowl.
Stir or beat until fully combined and smooth with no lumps; then fold in the mix-ins.
Scoop dough into 2-tablespoon balls and place them on a baking sheet lined with parchment or a silicone mat; you do not need to space them because the dough will be chilled.
Chill the dough on the prepared sheet for at least 30 minutes.
Preheat the oven to 350°F (175°C) and line a second baking sheet with parchment or a silicone mat.
Arrange chilled dough balls about 2 inches apart on the prepared baking sheet and bake 10–13 minutes, until they lose their glossy sheen.
Immediately after removing from the oven, press a few extra mix-ins on top if desired, then cool the cookies at least 10 minutes before transferring to a wire rack.