"My grandmother and mom used to make these every Christmas when I was little and my grandmother has since passed away and my mom lost the recipe. I looked for years and couldn't find anything that made cookies like those I remembered and then my aunt found her recipe! These are best after they've set overnight and become softer, moist and spongy, like cake. Separating the egg is a bit of a pain, but I think that may be what helps to give these such a different texture than your average sugar cookie. I sprinkle these with red and green sugar at Christmastime. Enjoy!"
Preheat oven to 350 degrees F (175 degrees C). Line a baking sheet with parchment paper.
Combine flour, baking soda, and salt in a bowl.
Combine sugar, butter, egg yolk, and vanilla extract in a separate bowl; beat with an electric mixer until creamy. Add flour mixture in 3 batches, alternating with buttermilk; beat dough well after each addition.
Clean beaters and beat egg white in a glass, metal, or ceramic bowl until stiff peaks form. Fold gently into the dough.
Drop tablespoonfuls of dough onto the prepared baking sheet.
Bake in the preheated oven until cookies just start to brown, 10 to 11 minutes.