In a medium bowl add sugar cookie mix, butter and egg. Mix well with a spatula until all combined (note: this can take some elbow grease, so you can use a hand mixer for the start on low to prevent overmixing).

Using a 1 tablespoon cookie scoop, drop dough balls into to fill the muffin tray (you will have enough for 36 cookie).

Bake for about 10-12 minutes, or until you see the edges starting to get a little golden brown.

Remove from the oven and using a tart tamper or a tablespoon lightly press into each cookie to make a small cavity (only go about half the way down, or less).

Allow them to cool for at least 15 minutes to ensure the frosting does not melt.

Once fully cooled remove cookie cups carefully, you may want to use a small spoon to loosen the edges (I like to use my tart tamper and turn it to loosen the cookie then tip the tray over to slide them out).

Then start your buttercream frosting by adding softened butter, powdered sugar and milk into the bowl of a stand mixer and mix on medium speed until smooth; stopping to scrape the edges down to ensure everything is smooth and consistent.

Transfer to a piping bag with any piping tips desired and decorate cookie cups.

If using, add any sprinkles.