Preheat oven to 400 degrees F. Line a baking sheet with parchment paper and set aside.
To make the streusel topping: In a small bowl, combine the flour, brown sugar, and butter. Mix together with your fingers until the butter is incorporated and the mixture resembles large crumbs. Put in the refrigerator until ready to use.
To make the scones: In a large mixing bowl, whisk together flour, sugar, baking powder, baking soda, and salt. Set aside.
In a small bowl, whisk together 1 cup of the heavy cream, egg, and vanilla extract.
Add the pieces of cold butter to the dry ingredients and lightly toss until all of the pieces are coated. Using a pastry cutter or your fingers, quickly cut the butter into the flour mixture until the mixture is pebbly. You want there to still be little chunks of butter, about pea size. Don't over mix.
Pour the cream mixture over the dry ingredients and use a fork to gently stir together. Don't over stir, the mixture will be wet and sticky. Gently stir in the blueberries. Still in the bowl, knead the dough a few times until the dough comes together.
Place the scone dough on a lightly floured surface. Gently pat and shape the dough into a 3/4-1-inch thick circle. Use a sharp knife to cut the scones into 8 even triangles. Place the scones on the prepared baking sheet and brush with the remaining heavy cream. Remove the streusel from the refrigerator and sprinkle the scones generously with streusel topping. Lightly press the topping into the scones. Place the scones on the baking sheet in the freezer for 20 minutes before baking.
Remove the scones from the freezer and place the baking sheet in the oven, on the middle rack, and bake for 17-22 minutes or until the tops and edges are golden brown and the scones are set but still slightly soft in the center. Remove from the oven and let the scones cool on the baking sheet for 5 minutes. Transfer to a wire cooling rack and cool completely.
While the scones are cooling, make the glaze. In medium bowl, whisk together the confectioner's sugar, 2 tablespoons milk, and vanilla extract. Whisk until smooth, if the glaze is too thick, you can add a little more milk.