Preheat the oven to 350°. Add the hazelnuts to a baking sheet and bake until the skins are darkened and some are easy to loosen, 10 to 12 minutes. Remove and set aside to cool slightly. Remove and discard as much of the skin as possible by rubbing with fingers, then coarsely chop the nuts.
Meanwhile, grate the cold butter on the large side of a box grater; chill.
In a large bowl, whisk the flour, granulated sugar, salt, and baking powder to combine. Add the butter, mixing briefly with fingers to distribute. Add the chopped hazelnuts and rosemary, tossing to coat.
In a medium bowl, combine the buttermilk and heavy cream, then slowly add the mixture to the dry ingredients, stirring with a spatula to incorporate. Break up the honeycomb into small pieces with your fingers and mix into the dough by hand to distribute throughout.
On a parchment-lined baking sheet that’s been lightly dusted with more einkorn flour, turn out the dough. Using floured hands, shape into a 12-inch-long, 1-inch-thick rectangle. Chill until firm, about 1 hour.
Preheat the oven to 400°. Using a sharp knife, make a diagonal cut crosswise through the dough every 3 inches to form 8 triangular scones. Separate the scones across the baking sheet, leaving at least 2 inches between each. Brush the scones with heavy cream and sprinkle each with about 1⁄4 tsp. coarse sugar.
Bake until lightly browned on the top and golden-brown on the bottom, 18 to 24 minutes. Remove and let cool 5 minutes, then transfer to a rack to cool completely.