Preheat oven to 400ºF. Line a baking sheet with parchment paper.
If currants are dry, soak them in warm water for 10 minutes, then drain.
In a large bowl, rub the lemon zest into the sugar to release oils.
Add flour, baking powder, baking soda, and salt; whisk to combine. Stir in the currants.
Grate the butter using a box grater and toss it into the dry ingredients. Alternatively, cut butter into pieces and mix with a pastry blender.
Add ¾ cup buttermilk and mix gently. If dough is dry, add more buttermilk, 1 tbsp at a time, until it comes together.
Turn dough onto a floured surface and shape into a 7-inch circle, about 1½ inches thick.
Brush the top with melted butter and sprinkle with sugar. Cut into 6 to 8 wedges and place on the baking sheet.
If possible, chill scones for 20 minutes before baking.
Bake for about 20 minutes, until tops are lightly golden.
Let cool briefly and serve with butter, lemon cream, or jam.