
Preheat oven to 220°C / 425°F. Line a baking sheet with parchment paper.
In a large bowl, whisk together flour, salt, baking powder, baking soda, and sugar.Add the very cold butter and cut it into the flour until pea-sized pieces remain. Do not overmix..
In a measuring cup, combine sour cream, milk, and vanilla. Keep cold until ready to use.Pour liquid into the flour mixture and gently fold with a spatula until the dough just comes together. Do not knead.
Toss the rhubarb with sugar and let sit for about 10 minutes. Drain off any liquid and pat dry.
Turn dough onto a lightly floured surface and shape into a rough rectangle.Fold the dough like a letter: top third down, bottom third up. Rotate 90 degrees, roll again, and repeat once more.After the first fold, scatter the rhubarb pieces over the dough. Continue folding as described.Pat dough into a thick round and cut into 8 scones.
Place on prepared baking sheet. Chill for 10–15 minutes if possible.Brush tops lightly with cream (optional).Bake for about 15 minutes until golden brown and risen.Let cool slightly and serve warm