Preheat oven to 350 degrees and line two baking sheets with parchment paper or Silpats.
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 egg and vanilla extract into the butter (don't worry if your mixture looks a little runny).
Add the muffin mix to the butter mixture and mix until just combined, taking care not to over-mix.
Scoop about 2 Tbsp of cookie dough (I use a #40 cookie scoop) onto the prepared baking sheets. Bake for 11-13 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 wire racks to cool completely.