Preheat the oven to 400F (200C) and line a baking sheet with a silicone baking mat or parchment paper.
1. In a small bowl, stir together the egg with the milk or cream.
2. In a large bowl, whisk together the flour, buckwheat, cornmeal, sugar, baking powder, and salt.
3. Using a pastry cutter, work in the cold butter until the pieces are about the size of corn kernels. (You could use an electric mixer or food processor instead.)
4. Add the egg mixture, stirring with a spatula, until the dough is moistened, then stir in the white chocolate bits and sour cherries.
5. On a lightly-floured surface, pat the dough into an 8-inch (20 cm) round. If it’s too wet and is very sticky, knead in a spoonful or two of flour on the countertop.
(The originally recipe called for 3/4 cup (180 ml) milk and cream, and my dough was very sticky, which may be the original intent, but I found it hard to work with. Slightly less than 1/2 cup, (115 ml) seemed right. Good thing making scones isn’t rocket science!)
6. Use a pastry scraper to divide the dough into eight wedges.
7. Brush the tops of each wedge with the a glaze made by stirring the egg yolk with the teaspoon of milk together with a fork. Dip the top of each scone in small bowl of coarse or regular sugar so they’re generously coated, then set each one right-side up on the baking sheet, evenly-spaced apart.
8. Bake the scones for 25 minutes, or until the tops are golden brown.