Preheat oven to 400°F (205°C). Line a baking sheet with parchment.

Grate the frozen butter on the large holes of a box grater and keep it chilled until needed.

Whisk together flour, baking powder, kosher salt, ground cinnamon, and brown sugar in a large bowl until uniform.

Toss the grated frozen butter into the dry mix. Use your fingers or a pastry cutter to quickly distribute; the butter should be pea-sized.

Fold in chopped bittersweet chocolate and toasted hazelnuts, reserving a few pieces and halves for the tops.

Make a well in the center and pour in cold heavy cream. Stir gently until a shaggy dough forms.

If dough is too dry, add up to 1–2 tablespoons more cream; avoid overmixing.

Tip dough onto a lightly floured surface and press into a 7–8 inch disk about 1 to 1 1/2 inches thick.

Chill in the fridge for 10–15 minutes.

Cut into 8 wedges with a sharp knife. Brush tops with beaten egg mixed with cream or milk, sprinkle with turbinado sugar, and press reserved chocolate pieces and hazelnut halves onto the tops.

Bake at 400°F (205°C) for 16–20 minutes until golden and slightly cracked on top.

Cool on a wire rack for at least 10 minutes.