Position a rack in the center of the oven and preheat to 400 degrees F. Line a baking sheet with parchment paper.
Whisk the cream, egg and milk powder in a liquid measuring cup. Pulse the flour, granulated sugar, baking powder and salt in a food processor until combined. Add the butter and pulse only until pea-size pieces remain. Transfer to a large bowl and make a well in the center. Pour the wet ingredients into the well and mix with a fork, incorporating the dry ingredients a little at a time until a shaggy dough forms (it's okay if the dough looks a little dry, just don't overwork it). Lightly knead the dough in the bowl until it just comes together.
Turn the dough out onto a lightly floured surface and pat into a 1-inch-thick rectangle (about 9 by 7 inches). Cut in half lengthwise, then cut in half again crosswise; you should have 4 equal rectangles. Cut each piece in half from corner to corner making 8 triangular wedges. Transfer to the prepared baking sheet and freeze for 10 minutes.
Brush the tops with cream and sprinkle with demerara sugar. Bake the scones until golden brown, 20 to 25 minutes. Let them cool on the baking sheet for 10 minutes, and then transfer to a wire rack to cool for 15 more minutes. Serve warm with softened butter and jam.
When measuring flour, we spoon it into a dry measuring cup and level off excess. (Scooping directly from the bag compacts the flour, resulting in dry baked goods.)
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