Melt-In-Your-Mouth-Shortbread

One of my least favourite types of cookies growing up was shortbread. But I love my husband and they're his favourite type of holiday cookie.
So I got a fantastic recipe for melt-in-your-mouth shortbread from my sister and I add a few loving twists, in this case, M&M's.
The key to melt-in-your-mouth-ness is replacing a quarter of the flour with cornstarch.
And so, cookie recipe #2 is:

M&M Shortbread Cookies

1 1/2 c. flour
1/2 c. cornstarch
1/4 tsp salt
 1 c. butter at room temp.
1/2 c. icing sugar
1 tsp vanilla
(Optional: M&Ms or chocolate chips or cinnamon)

Bowl: Whisk flour, cornstarch and salt. Set aside.

Second bowl: Cream butter til smooth. Add sugar and beat til smooth. Beat in vanilla.

Gently stir in flour mixture and M&Ms until incorporated.
Flatten into a disk. Wrap in plastic wrap and put in the fridge for at least an hour.
Cut into pieces and bake at 350 F for 10-12 minutes.