In a large mixing bowl, beat together softened butter and sugar until creamy. Add egg and vanilla and mix again until well combined, scraping sides of bowl as needed.
Next, alternate adding baking mix and milk until a thick dough forms. Depending on the flour blend/baking mix you use, this will take anywhere between 2.75 - 3.25 cups (as original recipe is written). I found that 3 was about the right amount. Cover and refrigerate at least 2-3 hours, preferably overnight.
Preheat oven to 375 degrees F (190 C). Scoop rounded 1 Tbsp amounts of dough and gently roll/form into balls. Place 2 inches apart on pan to allow for spreading.
Bake for 10-12 minutes or until slightly golden brown around the edges and no longer wet. They should be pretty puffy but not have a lot of color. Otherwise they'll become too dry.
Let set on pan for a few minutes, then use a spatula to transfer to a cooling rack. I topped mine with a few sprinkles, but these would also be excellent with a simple frosting of 2 cups powdered sugar + 1 Tbsp melted butter + 1-2 Tbsp milk OR my simple 4 ingredient macadamia coconut frosting. Store covered in an airtight container for several days. Freeze for longer term storage. Makes between 36-42 cookies (as original recipe is written).
*Prep time does not reflect 2 hours required for chilling dough. *We do not recommend subbing baking mix with flour as the cookies likely will not rise correctly without baking soda and baking powder. *Nutrition information is a rough estimate.