Preheat oven to 325°F. Line two baking sheets with parchment paper and set them aside.
In a bowl, cream the butter, shortening, and sugar until light and fluffy, this takes about 5-7 minutes. Then beat in the egg yolks and buttermilk.
Whisk the flour, baking powder, baking soda, and salt in a separate bowl; add it to the creamed mixture alternately with lemon juice and zest. Add the egg whites and beat again until smooth.
Drop the dough by rounded teaspoonfuls onto the prepared cookie sheets. Bake about 10 minutes, let cool 2 minutes, then transfer to wire racks to cool completely.
For the icing, combine all ingredients and whisk until smooth. Spoon or spread onto cooled cookies. Serve & enjoy!
Notes
These buttermilk lemon cookies will stay fresh for up to 5 days if stored in an airtight container at room temperature.Place them in an airtight container or freezer bag and freeze for up to 3 months. Thaw at room temperature before adding the icing and serving.