In a large bowl, combine the buttermilk, yogurt, sugar, vanilla extract, lemon juice, and salt.
Whisk together until the sugar is fully dissolved and the mixture is well combined.
Cover the bowl and refrigerate the Koldskål for at least 1 hour or until thoroughly chilled.
Serve the Koldskål cold, garnished with fresh mint leaves or lemon zest if desired. It can be enjoyed as a refreshing dessert or a light summer soup.
Enjoy your meal!