Adjust oven rack to middle position and heat oven to 450 degrees.
Sift first 5 ingredients into large bowl, or measure into workbowl of a food processor fitted with steel blade; pulse until blended. With pastry blender, 2 knives, or steel blade of a food processor, cut or process butter into flour mixture until mixture resembles coarse meal with a few slightly larger butter lumps.
If making by hand, make a well in the center of mixture and pour in heavy cream and eggs. Working quickly, add currants and blend ingredients together with a rubber spatula into a soft, slightly wet dough. If using a food processor, pour cream through feed tupe; pulse until dough just starts to gather into a rough ball (do not overprocess or scones will be tough). Turn dough onto a well-floured work surface.
Quickly roll dough to 1/2 inch thick. Use a lightly greased and floured 3-inch biscuit cutter to stamp dough with one decisive punch, cutting close together to generate as few scraps as possible. Dip cutter into flour as often as necessary to keep dough from sticking. Pus scraps of dough together so that edges join; firmly pinch edges with fingertips to make a partial seal. Pat this remaining dough to 1/2 inch thick; continue to cut 3-inch rounds. Place dough rounds 1 1/2 inches apart on a greased baking sheet. Brush with an egg wash and sprinkle with sugar, if desired. Bake until scones are lightly brown, 10 to 12 minutes. Serve immediately.