Preheat oven to 375°. Position rack in top half to third of oven.
Stir together first 6 ingredients. Add butter and work into dry ingredients with hands until dough is sandy but with pea-sized to slightly larger pieces of butter remaining.
Add blueberries and mix in gently with your hands or wooden spoon. (If using frozen blueberries do not defrost before adding to the mixture.) Add buttermilk. Dough should be completely moistened but not overly wet.
Sprinkle 2 to 3 tablespoons flour over dough and then scoop with muffin-size scoop into mounds on parchment-lined cookie sheet. (They do not have to be exact in size and shape). Leave room between each biscone, as they will spread.
Using a pastry brush, lightly brush the top of each mound with the egg wash, pressing down a little as you brush so tops of mounds flatten.
Sprinkle a pinch of granulated sugar on each biscone.
Bake for 10 minutes. Rotate pan and bake another 10 minutes. Biscones should have a marbled golden look and be firm to the touch.
Remove from oven and cool for at least 10 minutes.
While biscones are cooling, make glaze by whisking ingredients together. Add more powdered sugar as necessary so glaze is thick but still thin enough to pour.
Drizzle glaze over biscones and top with a large pinch of lemon zest.
Nancy Pedersen, owner of Sunflowers on the Square Kitchen and Bakery in Ventura, adapted this recipe from The Back in the Day Bakery Cookbook (Artisan, 2012) by Cheryl Day and Griffith Day.