Homemade Blueberry Biscuits

Preheat the oven to 425°F and line a sheet pan with parchment paper.
In a large bowl, whisk together flour, sugar, baking powder, cream of tartar, and salt.
Add 8 tablespoons of cubed butter and use your fingers to toss the butter and coat each cube in flour. Press each cube between your thumb and forefinger to shape it into a flat disc.
Add the blueberries to the flour and butter and toss well with your fingers to mix everything together.
In a medium bowl, whisk together the egg, vanilla, sour cream, and milk until smooth.
Pour the wet mixture over the dry ingredients. Stir with a wooden spoon until the dough is starting to come together.
Using your hands, fold the dough over itself several times, pressing the dough together and rotating the bowl in between folds, until all of the flour is incorporated.
Place a sheet of parchment paper over your countertop and transfer the dough in an elongated mound to the parchment paper. Press the mound into a 12" x 6" rectangle. Some berries will get mashed in the process but they will bake up jammy and delicious.
Cut the dough into 8 squares. Transfer each one to the prepared baking sheet.
Melt the remaining two tablespoons of butter and brush over the tops of the biscuits.
Bake for 20-22 minutes, until the tops are lightly browned and the bottoms of the biscuits are golden.
Serve immediately with jam, butter, cream cheese, or lemon curd. Cool completely before storing any leftovers in an airtight container.