In a mixing bowl, combine the flour, baking powder, baking soda and salt and mix thoroughly.
With a flat grater, shred the butter into the flour mixture, stopping occasionally to toss. You should have a coarse mixture with some larger chunks of butter scattered throughout.
Mix together the sour cream, maple syrup and half egg yolk (1 teaspoon of egg yolk). Add to the dry mixture and stir until blended. Add milk one tablespoon at a time just until mixture is moistened. You may not need all the milk. You don’t want it to be too wet.
Empty mixture onto the work surface (pastry mat, hopefully). Use your hands to flatten it and gently knead in floury bits of dough Press into a slab and fold over on itself once or twice, adding more flour if needed to keep it from sticking. Sprinkle chocolate over the slab and fold over on itself again. Shape your slab into a log that’s about 10 inches long and 1 ½ to 2 inches thick. Cut the log into 5 chunks
Put the chunks in the freeze while you preheat the oven.
Preheat the oven to 400 degrees F.
When oven is preheated, remove your scone chunks from the freeze and gently nudge them into rounds. Arrange them on a parchment lined baking sheet. If you are not planning on using the glaze, you may want to brush with some cream and sprinkle with sugar.
Bake at 400 for 10 minutes, then reduce heat to 350 and bake for another 18 to 20 minutes. They will brown considerably, but should not burn.
Let the scones cool for about 5 minutes while you make the glaze.
To make the glaze, put the 2 tablespoons of maple syrup and 1/4 teaspoon of vanilla in a 2 cup glass measure or in a bowl. Add the confectioners’ sugar 1 tablespoon at a time until glaze reaches desired consistency.
Spread a little maple glaze over the warm scones. If you want, you can sprinkle the tops with a little salt.