Prep: Preheat oven to 375°F. Line a baking sheet with parchment paper (you can also use a large cast-iron skillet). In a small bowl or large glass measuring cup, combine milk, vinegar, and egg. Beat well for about 30 seconds and set aside.
Dry Ingredients: In a large mixing bowl, add dry ingredients: 4 cups of flour, salt, sugar, baking soda, baking powder, and raisins/currants (if using). Whisk to combine.
Add Butter: Add cubed, COLD butter. Work the butter in with your fingers or a fork until pea-sized crumbs form.
Add Milk: To the flour mixture, add the milk/vinegar/egg mixture. [Note: Reserve about a tablespoon of the milk mixture to brush on top] Stir with a wooden spoon or fork until all of the dry ingredients are moistened. No need to overmix.
Form Dough: Flour your hands. Then, transfer dough to a lightly floured surface. Knead the dough 5-10 times or for about 30 seconds until all of the flour is incorporated. Again, no need to overmix. It will be a fairly sticky dough. If it's too sticky, add a bit more flour. Form the dough until a round loaf. This soda bread is meant to be "rustic"... meaning, it doesn't have to be a perfectly-shaped loaf!
Prepare to Bake: Place dough on your prepared baking sheet. Then, with a sharp knife, make a deep, 1/2 inch deep "X" on the top of the loaf. Brush the top with the reserved milk mixture.
Bake: Bake soda bread for 40-50 minutes or until the internal temperature is 190°. Transfer to a wire rack and allow bread to cool for 10-15 minutes before cutting into slices. Serve with lots of Irish butter or alongside Irish Stew!