In a large mixing bowl, combine the melted unsalted butter, room-temperature egg, vanilla extract, and room-temperature Neufchâtel cream cheese. Beat until completely smooth and combined.
Add the funfetti cake mix to the bowl in two additions, stirring just until the batter is combined and no dry streaks remain; avoid overmixing.
Fold in 1 to 2 tablespoons colored jimmies or sprinkles until evenly distributed.
Cover the dough and refrigerate for at least 1 hour to firm up.
Preheat the oven to 350°F (175°C) and line a baking sheet with parchment paper.
Scoop the chilled dough and roll into balls (about 1 to 1 1/2 tablespoons each). Place them about 2 inches apart on the prepared baking sheet.
Use the bottom of a measuring cup to gently flatten each ball into a disc, or leave them as balls for a taller cookie.
Bake for 9 minutes; the cookies should not brown—remove when the centers look set but still soft.
Let the cookies cool on the baking sheet for a few minutes, then transfer to a wire rack to cool completely.