Preheat oven to 400 degrees F (200 degrees C). Line a baking sheet with parchment paper.

Cover currants with warm water in a bowl and set aside to moisten.

Sift flour, baking powder, and baking soda in the bowl of a stand mixer. Using the paddle attachment at low speed, mix white sugar and salt into the flour mixture. Add all the unsalted butter to the mixer bowl and mix on low speed until butter cubes reduce to the size of small peas, about 30 seconds.

Drain currants and discard soaking water; mix currants, buttermilk, and lemon zest into the flour mixture on low speed just until the dough starts to hold together.

Turn dough out onto a lightly floured work surface and gently shape into a rectangle 18 inches long, 5 inches wide, and 1 1/2 inches thick. Brush dough with melted butter and sprinkle with sugar crystals.

Cut the dough in half crosswise with a sharp knife; cut each half into thirds, and cut each third diagonally to make 12 triangular-shaped scones. Place on the prepared baking sheet.

Bake in the preheated oven until scones are lightly golden brown, about 18 minutes. Eat warm.