In a big bowl mix the flour, sugar, baking powder, baking soda and salt. Just whisk them roughly so as to combine all the ingredients.
Cut the butter in small pieces, it should be cold, and add it to the flour bowl. Using a pastry blender or a couple forks cut into the butter and continue until the mixture resembles small peas.
Add the currants to the mixture and mix well. Make a well in the middle.
Pour two cups of the heavy cream over the flour mixture and using a spatula mix everything well together. If the mixture is too dry, add more heavy cream a tablespoon at a time. Turn the dough over onto a well floured surface and roll it out into a rectangle that's about 11"x7". Do not be tempted to knead the dough, you'll crush the currants. The rectangle should be about an inch in thickness.
Cut the dough, using a pastry cutter or a knife into 16 triangles.
Cover them up with plastic wrap and refrigerate them for a couple hours until the dough is firm.
Preheat oven to 375 F degrees.
Before placing them in the oven, mix the egg yolk, with the 1/4 cup of heavy cream and brush the scones. Sprinkle some sugar on top if you prefer.
Bake them in the oven for 30 to 35 minutes and then cool them off on a wire rack.