Preheat oven to 400°F. Whisk together flour, sugar, baking powder, baking soda, and salt in a large bowl.
Grate the cold butter using a cheese grater. This is an easy way to get it into small pieces to incorporate in the dry ingredients. If you do not have a cheese grater, you can just cut into small cubes. Blend the grated butter in with the flour mixture using a pastry cutter or 2 knives. The mixture should be clumpy and look like course crumbs. Mix in the blueberries and pecans.
In a small measuring cup, mix together the buttermilk and vanilla. Mix into the flour mixture until JUST incorporated. DO NOT over mix, this will make the scones too dense.
Put a small amount of flour onto a clean surface and knead the dough briefly. Shape the dough into a circle that is about 1 ½ inches thick. Cut the circle in half and then each half into 4-5 wedges (depending on how big you want your scones).
Put the wedges on a baking sheet covered with parchment paper. Brush the tops with a little bit of milk and then lightly sprinkle with sugar.
Bake for 22-25 minutes until tops are golden brown and toothpick inserted comes out clean. Cool on a wire rack.
You may not realize this but baking powder/baking soda actually expires pretty quickly. Use baking powder that is less than a year old since it has been opened. If you use older baking powder, it won’t be powerful enough to help leaven the scones. You can test it by stirring half a teaspoon of baking powder into a cup of hot water. It will immediately start to fizz and release carbon dioxide gas if it’s still fresh enough to use. Baking soda should be less than six months old and you can use the same test.
Use cold or even frozen butter. The cold butter coats the flour which creates crumbs that melt and steam, thus creating that flakiness you expect in scones.
Do not overwork the dough. Mix everything until just combined or you will have tough scones.
Leftover scones will keep at room temperature for 2 days or in the fridge for 5 days.