Place your mixing bowl, box grater, and butter in the freezer for 15 minutes before starting. Preheat oven to 450°F with a rack in the upper third. Line a baking sheet with parchment.
In the chilled bowl, whisk together the 2 cups all-purpose flour, 2/3 cups cake flour, 1 1/2 tsp baking powder, 1/2 tsp baking soda, 2 tbsp granulated sugar, and 1 tsp fine salt until evenly combined.
2 cups all-purpose flour, 2/3 cups cake flour, 1 1/2 tsp baking powder, 1/2 tsp baking soda, 2 tbsp granulated sugar, 1 tsp fine salt
Working quickly, grate 7 tbsp unsalted butter on the large holes of a box grater directly into the flour mixture. Toss with your fingers after every few passes to coat the butter pieces in flour. You should end up with a shaggy, uneven mixture with visible flat butter pieces throughout, that's exactly right. Use only your fingertips, not your palms.
7 tbsp unsalted butter
Toss 3/4 cup dark chocolate chips into the flour and butter mixture and distribute evenly before any liquid goes in.
3/4 cup dark chocolate chips
Combine 1 cups buttermilk and 1 tsp vanilla extract in a small bowl. Pour three quarters over the flour mixture and fold with a spatula using broad strokes from the bottom up. Add the remaining liquid a tablespoon at a time until the dough just comes together.
1 cups buttermilk, 1 tsp vanilla extract
Using a large cookie scoop or two spoons, drop heaping 1/3 cup mounds onto the prepared baking sheet, spacing about 2 inches apart.
Brush the tops generously with 2 tbsp heavy cream and sprinkle with 1/2 tsp flaky sea salt for topping.
2 tbsp heavy cream , 1/2 tsp flaky sea salt
Bake for 12–14 minutes until the tops are deep golden brown and the sides look set. Rotate the tray once at the halfway point. The high heat creates steam quickly which drives the rise.
Leave on the tray for 5 minutes before serving. Best eaten warm, ideally within an hour of baking. Pull apart rather than slice for the full reveal.