Preheat oven to 400°. Lightly spray an 8-well cast-iron wedge pan with baking spray with flour.
In a large bowl, whisk together flour, granulated sugar, and 1 teaspoon zest. Using a pastry blender, cut in cold butter until mixture is crumbly. Gently stir in ⅔ cup blueberries. In a small bowl, whisk together ⅓ cup buttermilk, egg, and vanilla. Make a well in center of flour mixture; add buttermilk mixture, stirring until moistened. Add remaining 2 tablespoons buttermilk, if needed. (Dough will be sticky.)
Turn out dough onto a lightly floured surface. Using floured hands, pat dough into a 7-inch circle. Cut into 8 wedges. Place dough in prepared wells, pressing into place.
Bake until golden brown and a wooden pick inserted in center comes out clean, about 18 minutes. Let cool in pan for 10 minutes. Run a knife between shortcakes and edges of pan; gently remove. Let cool on a wire rack for 30 minutes.
In a small bowl, whisk together confectioners’ sugar, lemon juice, and remaining 1/4 teaspoon zest. Drizzle glaze over shortcakes. Serve with Chantilly Whipped Cream and remaining 3 cups blueberries. Garnish with zest, if desired.
Chantilly Whipped Cream
Place a medium bowl and mixer beaters in refrigerator until very cold.
Whisk cream cheese until smooth. Add cream, vanilla and sugar to chilled bowl, and beat with a mixer at high speed until thickened. Serve immediately.