Combine 1/2 cup buttermilk and baking soda in a small bowl. Set aside until step 5.
In a stand mixer, cream together the butter and sugar.
Add eggs, salt, vanilla and 1/2 cup buttermilk. Mix well.
Add 3 cups of flour and the baking powder. Blend until combined.
Add the buttermilk/baking soda mixture and the last 3 cups of flour. Mix until the flour is completely incorporated.
Wrap the cookie dough in plastic wrap and gently press it into a disc. Be sure it is wrapped tightly and refrigerate the dough overnight.
When you're ready to roll out the dough, preheat the oven to 350 degrees.
Sprinkle a pastry mat with flour. Divide the dough in half and roll out one-half of the dough, sprinkling flour on top of the dough as needed.
Cut out the dough in the shapes you'd like. Place the cookies on a nonstick baking mat.
Bake the cookies at 350 degrees for 9-10 minutes, removing them before they are browned.
Repeat with remaining dough, rerolling as needed.
Allow the cookies to cool, the frost them with buttercream frosting and sprinkles, if desired.
Store in an airtight container.
*I haven't had much success with freezing this cookie dough, but the cookies do freeze well after they are baked. Wrap the cookies tightly (preferably before frosting). When defrosting the cookies, allow them to come completely to room temperature before taking the cookies out of the packaging.