Using a hand or stand mixer, beat butter and sugar together in large bowl.
Add the flour about 1/2 cup at a time and mix until well blended.
Add the cranberries and lemon zest and mix just until combined.
Place half the dough onto a large sheet of plastic wrap and slowly roll into a log. The dough is pliable at this stage so you can manipulate and shape it as you need within the plastic wrap.
Repeat with remaining dough.
Place the dough in the refrigerator for at least one hour or overnight.
When it's time to bake, preheat oven to 350 degrees.
Remove plastic wrap from dough and slice into rounds about 1/4 inch thick.
Carefully place dough slices on cookie sheets lined with silpats or parchment paper.
Bake for 12-18 minutes. 12 minutes will give you a softer shortbread texture, while 18 will give you a crispy shortbread.