Use a food processor fitted with a shredding blade to grate the frozen butter as you would cheddar cheese.
Mix together the oats, flour, currants, sugar, baking powder, baking soda, and salt.
Separately, whisk the egg into the heavy cream; take out about two tablespoons of this wet mix and set aside.
Pour the remainder into a center well of the dry mix and toss from underneath to blend, trying not to touch it too much, until the flour works almost completely in.
Turn out onto a parchment lined baking sheet and pat into a disk one inch thick, touching up the sides.
Use a chef's knife to cut the dough into wedges.
Brush the top with the reserved egg/cream, then sprinkle with sparkling sugar.
Bake for 25 minutes, then take out of oven, score the cut lines again with a chef's knife and separate the scones using a pie server.
Return to oven and bake for 8-10 minutes more, until nicely browned.