Combine flour, cornstarch, and salt in a medium bowl and set aside.
In a food processor or blender jar, pulse cranberries and 1/4 cup powdered sugar together until cranberries are about the size of coarsely chopped nuts. The powdered sugar will keep them from getting too clumpy, but don't pulse too long or you will have puree. Set chopped cranberries aside.
With an electric mixer, beat butter on high until it is very light and fluffy - at least 5 minutes. Beat in remaining 3/4 cup powdered sugar and beat until vey smooth and fluffy. Add vanilla and zest and mix to thoroughly combine.
Mix in chopped cranberries.
Add flour mixture and mix on low just until dough forms a ball on comes away from the sides of the mixing bowl.
Remove dough from bowl and form into two logs about 1 1/4" - 1 1/2" in diameter. Tip: Two shorter logs are much easier to work with than one long log.) Chill 1 hour.
If you just want to keep things simple and make shortbread rounds, leave the dough-logs in the fridge for two hours and proceed to "Baking" steps.
How to form Heart-shaped Shortbread Log
After one hour, remove prepared dough logs from refrigerator.
With the shortbread dough still in plastic wrap, shape the bottom point of the heart by resting the log on a cutting board and using the back of a spatula perpendicularly against a cutting board to form a point. You will need to rotate the log from one side to the other a couple of times to get it all even. When you have the bottom point the way you want it, use a large skewer or chopstick to form the indentation opposite the point. I spend a little time hand-shaping the curves so they look like they should.
If your heart doesn't look the way you want it, just round it up and pop it back into the fridge for about 10 minutes; then you can get a fresh start.
Once your Heart-Log is shaped the way you want, pop it back into the fridge for at least another hour before slicing.
Preheat oven to 350° F [175° C].
Using a clean, sharp knife, slice cookies 1/4" - 3/8" thick and place on a cookie sheet lined with parchment paper or a Silpat mat.
Arrange shortbread slices on baking sheet about 3/4" apart. Bake in preheated oven 8-9 minutes, or until just firm. Watch carefully, and do not allow edges to brown.
Can't find Meyer Lemons? Here's a couple of handy substitutions: One Meyer Lemon ➢Juice = 1 tablespoon lemon juice + 1 tablespoon tangerine or orange juice | Zest = 1/2 teaspoon tangerine or orange zest + 1/2 teaspoon lemon zest
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