Mix together all streusel ingredients except butter. Using your fingers or a pastry cutter, cut the butter in until fully incorporated. Refrigerate while you prepare the rest of the ingredients.
Combine flour, sugar, baking powder, baking soda, and salt. Using your fingers, a pastry cutter, or a food processor, cut butter in until there are pea sized pieces of butter mixed into the flour mix. Having some larger or smaller pieces are fine.
Whisk together buttermilk and egg with 1 teaspoon vanilla extract. Add to the dry ingredients. Using a silicone spatula or danish dough whisk, bring together into a cohesive dough. Let the dough chill in the refrigerator for 30 minutes.
When your dough is nearly done chilling, preheat oven to 375°F. Butter or nonstick spray an 8 inch by 4 inch loaf pan. Lay a cut piece of parchment paper in the pan, so that the edges hang over the sides. These will act as handles to remove your loaf after baking.
Turn dough out onto a floured surface. Roll the dough into a 14 inch by 6 inch rectangle using a rolling pin. Cut into 12 even pieces. Set one piece aside to cap off our loaf later on.
Spread about 1 teaspoon of blueberry jam on each piece and sprinkle streusel over the top. Use your fingers to lightly press the streusel into the jam so that it sticks. Press 4 to 5 blueberries on top of each piece.
Stack four of the pieces on top of each other, and turn sideways to place the pieces in your prepared loaf pan. It's like building a sideways lasagna.
Repeat with the remaining layers, topping the final stack with the plain piece we set aside earlier. Brush exposed edges of scone dough with an egg that has been whisked with a teaspoon of water. Sprinkle with any remaining streusel and press remaining blueberries between layers, so they poke out a bit. Sprinkle top with coarse sugar, if desired.
Bake at 375° for 50 to 60 minutes or until a thermometer placed in the center of the loaf reaches 195°. Let cool for at least 30 minutes.
Whisk together powdered sugar, milk, and vanilla until completely combined. Drizzle over cooled loaf, and use parchment handles to remove it from the pan.
Slice and serve warm. Store leftovers tightly wrapped in plastic wrap. Can be reheated by microwaving for 10-15 seconds.