In a large mixing bowl, add the 2 ½ cups of flour, the sugar, leavening agents, and salt.
Then grab the 6 tbsp of cold butter from the fridge and pinch together (like the streusel) the flour mixture with the butter. It will take a minute or so, but the result will leave you with pea-sized butter chunks in the flour mixture.
Next, in a medium liquid measuring cup, add 1 cup of cream, followed by the egg and vanilla extract. Whisk with a fork until thoroughly combined.
Slowly add the wet ingredients into the dry flour mixture. Gently stir with a fork until it is barely mixed. There should still be plenty of flour streaks.
Then you are going to take the washed and dried blueberries and add 1 tbsp of flour to coat them before adding into dough.
Carefully stir in the blueberries until barely combined. It's helpful to use your hands at this point. Dump the scone mixture on a floured countertop. Use your hands to form 2 even dough balls.
Take each dough ball and flatten with your hands until you reach a 1-1½ inch thickness. Slice each flatten dough patty into 6 scones. You will be left with 12 scones.
At this point you will need to preheat your oven to 400°F. Line 2 baking sheets with parchment paper. Place 6 scones on each baking sheet.
Brush each scone with the additional 2 tbsp of cream. Then evenly distribute the streusel mixture among the 12 scones.
Pop your baking trays into the freezer for 20-30 minutes. You can do this in shifts or snuggle all 12 scones onto 1 tray to freeze efficiently. Just make sure to leave a few inches around each scone to allow them to rise as they bake.
Once the scones have chilled, place them in the preheated oven and bake for about 14 minutes.