Whisk dry ingredients. Preheat the oven to 400°F. In a large mixing bowl, whisk together the gluten-free flour, sugar, baking soda, baking powder, and salt.
Add cold butter. Cut the butter into small cubes and add it to the flour mixture. Toss to coat it in the flour, then use your hands to work it into the dry ingredients until it’s in roughly pea-sized pieces.
Fold in the currants. Add the currants to the flour mixture and stir until evenly combined.
Add the buttermilk mixture. In a medium-sized bowl, whisk together the buttermilk and egg. Make a well in the center of the flour mixture and pour in the buttermilk and egg. Stir with a spatula or wooden spoon until the wet and dry ingredients come together into a shaggy dough.
Shape the dough. Transfer the dough to a lightly floured surface and knead to form it into a 7-inch round ball. It should be soft, but not too sticky.
Transfer the dough to a skillet. Add the dough to a buttered cast iron skillet. Using a very sharp knife, score a ½-inch deep X on top of the loaf. Use any remaining buttermilk mixture to brush on top.
Bake. Place the skillet in the oven and bake until golden brown, 45 to 55 minutes. You’ll know the loaf is ready if it makes a hollow sound when you tap the bottom, or when an instant-read thermometer reads 190°F (88°C). Let the soda bread cool for 10 minutes in the skillet before transferring it to a wire rack to continue cooling. Serve warm or at room temperature.