Pour the flour, sugar, baking powder, salt, and baking soda into a large bowl. Whisk to combine. Add the blueberry butter and using your thumb and index fingers, rub the butter until the flour mixture. Make sure to be quick with your movements, so you don't melt the butter.
3 cups all-purpose flour, 2½ tsp baking powder, ¼ cup granulated sugar, 1½ tsp kosher salt, ½ tsp baking soda
Crack one egg into the buttermilk and whisk to combine. Pour the mixture into the flour mixture and stir until a shaggy dough forms. Turn the dough onto a floured surface and lightly knead it into a ball. Again, be quick and light with your movements.
½ cup blueberry butter (see above), ¾ cup buttermilk
Roll the dough out to a rough rectangle about 1/2 an inch thick. Fold the dough like a letter and give it a quarter turn. Repeat until the dough returns to its original orientation.
Trim the edges of the dough and cut it into 8 squares. Transfer the biscuits to a large baking sheet lined with parchment paper or a silicone mat. Gather up the dough scraps and roll them out. Cut out two more squares and add them to the baking sheet. Transfer the baking sheet to the freezer and chill for 1 hour.
While the biscuits are chilling, preheat the oven to 400°F.
Once the biscuits are sufficiently chilled, crack the remaining egg into a bowl and add the water. Whisk to combine. Brush the biscuits with the egg wash and sprinkle with the demerara sugar. Transfer the biscuits to the oven and bake for 20 minutes or until golden, rotating the pan once halfway through.
¾ cup buttermilk, 2 large eggs, 1 tbsp water
Transfer the finished biscuits to a cooling rack and let cool completely before serving.