Preheat oven to 350 degrees and line two baking sheets with parchment paper.
In the bowl of a stand mixer or in a large bowl with a hand mixer, cream the butter until light and fluffy, 2-3 minutes (if you don't have a mixer, you can use a bowl and wooden spoon/spatula).
Mix whole egg and egg white into the butter until combined (don't worry if your mixture looks a little runny/clumpy).
Add the Funfetti cake mix to the butter mixture and mix until just combined, taking care not to over-mix.
Scoop about 2 tablespoon of cookie dough (I use a #40 cookie scoop) onto the prepared baking sheets. Bake for 10-12 minutes, until cookies are just turning golden around the edges, but are still soft. Let cookies cool in the pan for a couple minutes before moving to a wire rack to cool completely.