Place a rack in the center of your oven and preheat the oven to 400 degrees F. Generously grease a standard 12-cup muffin tin with nonstick spray.
In a large bowl, stir together the whole wheat flour, all-purpose flour, granulated sugar, baking powder, baking soda, salt, and caraway seed. In a separate bowl, whisk together the egg, milk, butter, and Greek yogurt. Make a well in the center of the dry ingredients, then pour the wet ingredients into it. Stir just until combined. Fold in currants.
Divide the batter among the muffin cups and sprinkle the tops with sparkling sugar, if using. Bake the muffins for 13 to 15 minutes, until a toothpick inserted in the center comes out clean. Place the pan on a wire rack and let the muffins cool in the pan for 5 minutes, then remove the muffins to a rack to continue cooling. Enjoy warm, topped with butter, jam, a slice of cheese, or on their own.