Preheat oven to 400 °F. Sift together dry ingredients in a medium bowl. Using any method you prefer, cut the cold butter into the dry ingredients until flour resembles coarse meal, or use the following tip: The "grate" method: grate a block of cold butter against the large holes of a box grater. Toss the butter ribbons with the flour mixture, and then use two knives to cut the flour-coated ribbons into 1/4-inch pieces. In a separate bowl, whisk together the sour cream, egg yolk and vanilla extract until blended. Add this to the flour-butter mixture and stir with a fork until dough forms a cohesive ball. Use a spatula to get the dry bits fully incorporated. (It may not seem to have enough liquid at first, but the dough will eventually come together.). Place the sticky dough onto a parchment-lined (or ungreased) baking sheet and pat into a disk about 1/2-inch in height. Cut the dough into wedges separate a little. Or scoop dough into a mini scone pan if available.
Bake for 15 minutes or until golden brown on top.
Prepare glaze while scones are baking.
GLAZE: Place powdered sugar in a medium bowl, and add milk and vanilla extract, mixing vigorously until smooth and runny. Use a pastry brush to apply a thin layer of glaze over the hot scones. Brushing rather than drizzling gives a smoother appearance and requires less glaze. Serve immediately. Store cooled scones in an airtight container.